preprint (0) 1 1
ROMAIN: Towards a BFO compliant
Reference Ontology for Industrial
Mohamed Hedi Karray a,*, Farhad Ameri b, Melinda Hodkiewicz cand Thierry Louge d
aLGP-INP-ENIT, Université de Toulouse, Tarbes, France
bDepartment of Engineering Technology, Texas State University, TX, U.S.A
cFaculty of Engineering and Mathematical Sciences, University of Western Australia, Australia
dCALMIP, Université de Toulouse-CNRS-INPT-INSA-UPS, UMS 3667, France
In this paper, we present a domain-speciﬁc, open access, reference ontology (ROMAIN) for the maintenance
management domain. We use a hybrid approach, based on a top-down alignment to an open source top-level ontol-
ogy, the Basic Formal Ontology (BFO), and a bottom up focus on classes that are grounded in maintenance prac-
tice. We constrain the scope of the ontology to the classes that are unique to the maintenance management practice,
such as maintenance strategy, degradation, and work order management, rather than modeling the entire domain
of maintenance. This approach reduces the scope of the development task and enables reasoning to be tested at a
manageable scale. ROMAIN provides a unifying framework that can be used in conjunction with other BFO com-
pliant sub-domain ontologies, such as planning and scheduling ontologies. The proposed ontology is validated using
real-life data in the context of a use case related to evaluating the effectiveness of maintenance strategy.
Keywords: Basic Formal Ontology, industrial maintenance, maintenance management
The competitiveness and responsiveness of manufacturing ﬁrms depend, in part, on how effectively
they maintain their assets and ensure the availability of their resources. Maintenance is deﬁned as “the
actions intended to retain an item in, or restore it to, a state in which it can perform a required function”
(IEC, 2016, 2011). Almost all assets of any value need maintenance and, as a result, there has evolved
organizational structures and processes to manage maintenance with theory and practice to support its
delivery. Maintenance management practices are documented in Standards (IEC, 2016), textbooks (Kelly,
2006, 1997; Palmer, 1999) and by professional societies (SMRP, 2009; GFMAM, 2016).
Data are key to the effective and safe maintenance of assets over their life cycle. This data is generated
by, and drawn from, various sources such as maintenance management systems, design documentation,
*Corresponding author. E-mail: firstname.lastname@example.org.
0 – authors (preprint).
2H. Karray et al. / ROMAIN
original equipment manufacturer manuals, process control systems, third party service providers, risk
management assessments and failure investigations (Morello et al., 2010), to name just a few. However,
due to the heterogeneity of data sources and diversity of data types, unlocking the real value of data and
discovering the useful patterns of knowledge embedded in the maintenance data has always presented a
major challenge (Bendell, 1988; Karray et al., 2011; Knapp and Hasibether, 2011). Ontologies can effec-
tively address this challenge by semantic annotation, integration, consistency checking and organization
of data (Karray et al., 2010).
Nowadays most asset-owning organizations will have a Computerized Maintenance Management Sys-
tem (CMMS) whose data schemes conform to a common understanding of work ﬂows in maintenance
practice. As a result, the way maintenance work is managed across different sectors such as defense, man-
ufacturing, and service, is remarkably consistent. The details of the work vary with the asset type but
the modeling entities and workﬂow processes are common. This makes maintenance an ideal candidate
for an ontology that can have wide-spread application as the classes used to describe the maintenance
management process and information are commonly used by the maintenance community.
Over the last 25 years, groups in industry, academia, and on standards committees have developed on-
tologies for engineering and asset management applications. These works include maintenance concepts
but do not focus on maintenance management practice and processes. As a result, there has been limited
uptake by maintenance practitioners (Hodkiewicz, 2018). There is a need for a reference ontology in the
maintenance domain that can serve as the overarching semantic model connecting various application
ontologies related to maintenance. The work proposed in this paper is positioned to fulﬁll this need.
In this paper, we present a reference ontology for industrial maintenance management. The reference
ontology developed in this work uses a top-level ontology. The development of ROMAIN started with
identifying the general terms most commonly used in maintenance practice and continued with the rep-
resentation of more complex entities in the domain. Although the ontology, in its current state, covers
most of the key notions speciﬁc to the maintenance domain, it can be extended in order to take account of
the technological and scientiﬁc advances. The rest of the paper is organized as follows. We start with the
motivation involving a description of the maintenance management domain. An overview of the current
state of the art in ontology practice is provided in section 2, next followed by a critical review of previous
work related to maintenance in section 3. Section 4 is devoted to discuss best practices of ontology de-
velopment. The development approach for ROMAIN is then described and the structure of the ontology
is presented in sections 5 and 6. We evaluate ROMAIN against its design principles and validate it with
a use case related to measuring the effectiveness of maintenance strategy in section 7. Finally, we discuss
challenges and future directions.
2. Background of Maintenance Management
This section provides the deﬁnitions and the necessary background information related to some of the
important entities in maintenance management. As all assets are likely to fail at some time, there exists
a business function to maintain the asset so that the production process or service provided by the asset
can continue. This function is called maintenance management. The maintenance management process
follows a process such as that shown in Figure 1. Key steps in this process are developing maintenance
strategy and life plans, administration steps involving planning, procurement and scheduling, task exe-
cution, administration associated with cost and performance assessment and strategy optimization. The
steps in this process that are speciﬁc to maintenance are shown as shaded. Non-shaded steps contain pro-
cesses, such as planning, scheduling and cost management, that are commonly used in other functions in
Maintenance management is not a core subject in many engineering curricula and the maintenance
world is largely impenetrable to those who have not worked in it as it has its own tribes, language and
customs (Reiman and Oedewald, 2004). With this in mind, core maintenance and work management con-
cepts are explained in the following section. Italics are used to identify concepts that will be represented
H. Karray et al. / ROMAIN 3
Fig. 1. Maintenance practitioner view of the management process, adapted from Coetzee (2004)
as classes in the domain ontology. These classes are drawn from maintenance practitioner texts and refer-
ences (Stuber, 2016; Coetzee, 2004; Kelly, 2006; Palmer, 1999).
Each asset has a primary function, this describes the main reason(s) for owning or using the asset. It
may also have a set of secondary functions due to the need to fulﬁll regulatory requirements and require-
ments concerning issues of protection, control, containment, comfort, appearance, energy efﬁciency, and
structural integrity (SAE, 2011). The maintenance action performed on an asset depends on the function
to be maintained and the consequence of a functional failure. Since the 1970s, the most common approach
to developing an asset’s maintenance plan is a process called Reliability-centered Maintenance (RCM).
In RCM, the emphasis is on identifying each functional failure and selecting an appropriate maintenance
strategy to manage each failure mode. The output of the RCM process is the identiﬁcation of mainte-
nance strategies (IEC, 2011) and associated maintenance tasks for each maintainable item. A collection
of strategies and the associated tasks over the life cycle is called a maintenance plan. The concepts de-
scribed above for function, maintenance strategy, maintenance plan, and maintainable item are speciﬁc
to maintenance and need to be represented in a maintenance domain-speciﬁc ontology.
There are six types maintenance strategies in RCM. These are a) condition-based, b) ﬁxed interval
restoration (also called scheduled restoration), c) ﬁxed interval replacement (also called scheduled dis-
card), d) failure-ﬁnding, e) run-to-failure, and f) design-out (also called modiﬁcation) (Moubray, 2007).
The strategy selected depends on failure behaviour and the consequence of failure. Following strategy se-
lection, a speciﬁc maintenance standard work procedure and interval are developed and this information
is stored in the CMMS. Some of the strategies, except run-to-failure and modiﬁcation, are preventative or
predictive in nature. Individual preventative tasks are triggered at the appropriate time by the CMMS and
each instance is represented by a maintenance work order record (MWO). Corrective work is triggered by
afailure event on a component resulting from a non-conformity. The observed failure is documented using
amaintenance work notiﬁcation as part of the work identiﬁcation stage. Once work is identiﬁed, either
through failure events or actions triggered by maintenance strategies, then the work is planned and sched-
uled. Planning involves the procurement of parts and the organization of personnel, tools, other resources
and the availability of the asset. Once complete work can be scheduled. Finally, the value-adding step is
when the work is executed by maintenance personnel. After execution work there should be analysis of
the quality and effectiveness of the maintenance work and opportunities for improvement identiﬁed. We
4H. Karray et al. / ROMAIN
suggest that the processes of planning, scheduling, and execution are generic to many industrial processes.
As a result they do not need to be developed speciﬁcally for this maintenance ontology. Instead existing
ontologies that are aligned to the same top-level ontology can be used to represent these processes.
3. Ontologies in Maintenance
Maintenance management can be viewed through two different lenses. The ﬁrst is the view of mainte-
nance practitioners of the work management process described in the previous section. In this view, shown
in Figure 1, maintenance is a process in which work on many assets is identiﬁed, planned, scheduled, ex-
ecuted and analyzed. This view is seldom apparent to operators and owners and is almost entirely absent
in maintenance ontologies built to date.
The alternate view considers the requirements of a speciﬁc asset, how it should be maintained over its
life, its reliability and the risk that its failure presents to the organization. This view, shown in Figure 2,
takes an asset life cycle perspective that is primarily of interest to operators and owners of assets (ISO,
2014), rather than maintenance management practitioners. Ontologies that mention maintenance usually
have this perspective (Ebrahimipour and Yacout, 2016; Karray et al., 2012; Matsokis and Kiritsis, 2010;
Otte et al., 2018). Examples include the Product Life Cycle Management Ontology proposed by Matsokis
and Kiritsis (2010) and a more evolved version of this ontology focusing on the operating phase of the
life cycle and integrating concepts related to the maintenance ﬁeld such as alarms, events, and activities
(Matsokis et al., 2010). Likewise, Karray et al. (2012) proposed IMAMO (Industrial Maintenance Man-
agement Ontology) as a modular ontology that decomposes the maintenance ﬁeld to according stakehold-
ers’ views. Another notable ontology is CDM-Core, a publicly available manufacturing ontology, devel-
oped according to condition monitoring data model from ISO13372 (ISO, 2012) to semantically annotate
sensor data. The CDM-Core work focuses on an asset’s performance rather than on maintenance work
management but has useful descriptions of quality measures and a detailed veriﬁcation process (Mazzola
et al., 2016). It is also to note that most of these ontologies do not use any axioms which prevents them
from being used effectively for reasoning and consistency checking services.
Ruiz et al. (2004) developed a semi-formal ontology that represents products, roles, activities, processes,
workﬂow, and actions related to software maintenance. Software failure modes and maintenance processes
are different from those of physical assets. Therefore, this work is not considered further here. Other
ontologies in the maintenance domain have been built for speciﬁc use cases such as for failure modes and
effects analysis (Ebrahimipour et al., 2010) and maintenance on a pneumatic values (Ebrahimipour and
Fig. 2. An asset-centric view of maintenance, commonly represented as a phase in one part of an asset’s life cycle
None of the ontologies mentioned above use a top-level ontology. We observe that when individual
groups develop domain ontologies that are focused on their speciﬁc local needs, the resulting ontolo-
gies are incompatible, isolated, and non-sharable. These problems are widely recognized in the ontology
sphere, where it is commonly assumed that they can be (or even are already being) addressed by the use
of sophisticated ontology mappings or similar technology (Arnold and Rahm, 2014). In our experience,
ontologies developed independently and on the basis of distinct sets of ontology development principles
(or even on the basis of no principles at all) remain in almost every case unmapped. Moreover, even where
mappings are created, they are rarely able to be updated in the needed ways because the ontologies on both
sides of the mapping-relation are themselves being changed independently and on asynchronous cycles.
H. Karray et al. / ROMAIN 5
The result is incremental decay of almost all existing mappings. We believe that the use of a top-level
ontology as well as a set of shared principles provide a common overarching framework that ensures con-
sistent development of ontologies across communities of researchers and ontology developers such as the
case of OBO Foundry in the biomedical domain (Smith et al., 2007).
There were some efforts related to development of the top-level ontology for process plant design engi-
neering in the 1990s. This was driven by the oil and gas and chemical industries. The resulting ontology is
documented in the ISO Standard 15926 (ISO, 2003) and described by Batres et al. (2007). The ISO 15296
Standard resulted in a number of applications relevant to the process plant design phase (Batres et al.,
2007) but has not resulted in the development of a domain-speciﬁc ontology for maintenance management
As part of manufacturing ontology suite, a Basic Formal Ontology (BFO; Arp et al., 2015) compliant
ontology of maintenance was proposed in the scope of CHAMP project (Otte et al., 2018). Because the
main focus of CHAMP project is on manufacturing processes and entities, the coverage of maintenance
in this ontology is limited and the maintenance terminology used drawn from less technical and reliable
sources such as Wikipedia. It is not apparent that there has been a review of the maintenance classes
against maintenance standards and practice.
4. Ontology Development Methodology
Ontologies have been widely used for knowledge representation as they provide a common vocabulary
for modeling a speciﬁc domain by capturing knowledge in a structured and formal way. Gruber (1995)
deﬁned ontology as ‘an explicit and formal speciﬁcation of a shared conceptualization’. The quality and
viability of an ontology depends on the methodology followed for ontology development. Also, the re-
quirements for an ontology is affected by how the ontology is positioned in the stack of ontologies ranging
from foundational ontologies to application ontologies. In this section, an overview of different types of
ontologies is provided to set the stage for positioning the proposed maintenance ontology as a reference
ontology. Best practices for ontology development are presented next. One of the novelties of the proposed
ontology is conforming with a top-level ontology. Therefore, the beneﬁts of using top-level ontologies are
discussed toward the end of this section.
4.1. Types of Ontologies
Several classiﬁcations are presented in the literature to differentiate ontologies based on their level of
formality and generality. Top-level ontologies are generic ontologies that can be viewed as ontologies de-
scribing the top-level classes that can be used for building other ontologies. They enable interoperability
between domain ontologies that incorporate the same top-level ontology (Noy, 2004), thus facilitating
data integration and knowledge reuse. Examples of foundational ontologies include the Suggested Upper
Merged Ontology (SUMO; Pease, 2006), the Descriptive Ontology for Linguistic and Cognitive Engineer-
ing (DOLCE; Masolo et al., 2002), the Basic Formal Ontology (BFO; Arp et al., 2015), the Uniﬁed Foun-
dational Ontology (UFO; Guizzardi and Wagner, 2010), Yet Another More Advanced Top-level Ontology
(YAMATO; Mizoguchi, 2010) and ISO15926 (ISO, 2003). A comparison between some of the previously
mentioned top-level ontologies can be found in the work by Mascardi et al. (2007). In contrast, domain
ontologies are only applicable to speciﬁc domains with a speciﬁc point of view. They describe the vocabu-
lary related to a particular area of knowledge (such as biology, environmental sciences, maintenance, etc.)
and use classes introduced in the top-level ontology. A reference ontology brings together the existing do-
main ontologies by providing canonical and precise deﬁnition of the common entities used in the domain.
Reference ontologies focus on rich and axiomatic representation of the intended meanings of common
terms in a domain (Andersson et al., 2006). For this reason, reference ontologies often go through several
rigorous quality checks according to principled methodologies. Reference ontologies are typically non-
proprietary and non-implementation-speciﬁc and can be adopted as a sort of standard in a community to
6H. Karray et al. / ROMAIN
create application ontologies (Falquet et al., 2011). Application ontologies are specializations of reference
ontologies and they are created to accomplish some speciﬁed local tasks or applications.
4.2. Best practices in ontology building
According to Simperl et al. (2010), most ontology development works are based on an unprincipled
approach to ontology engineering which results in non-interoperable and inconsistent ontologies . For
the purpose of construction of any ontology, many works (Arp et al., 2015; Arpirez et al., 1998; Gruber,
1995), elaborate clearly principles to respect in order to design useful ontologies. Many of these principles
turn on the fact that, since ontologies are built to be shared between actors, it is beneﬁcial if the ontologies
used in a given domain share a common upper layer of well-deﬁned terms (Smith, 2006). This upper layer
should be domain neutral, since its aim is to represent the most general categories of entities and the most
general relations within and between them (Arp et al., 2015). The categories taken together must provide
the means to deﬁne classes whose instances can be found in any realm of reality. According to Smith and
Ceusters (2010), a reference ontology should cover the terminological content of the settled portions of
given scientiﬁc discipline. In practice the ontology building process is more efﬁcient if key concepts are
deﬁned according to a top-level ontology ﬁrst rather than developing the ontology from the bottom up
and then trying to align it retrospectively to a top-level ontology. It is also important that each ontology
should have an is-a hierarchy having the structure of a directed rooted tree. Terms in the ontology should
be deﬁned in a consistent manner. Finally, the ontology must be available to be used by all potential users
with few constraints.
4.3. Value of alignment to a Top level Ontology
When multiple maintenance ontologies are developed in relative isolation from each other by different
communities, disconnected information silos will be created. To ensure that different application ontolo-
gies employ comparable and mutually-understandable deﬁnitions and knowledge constructs, they need to
share a common top-level formal ontology. The use of a top-level ontologies facilitates the alignment be-
tween several domain ontologies and promote greater data interoperability through conformity to a com-
mon semantic model. Top-level ontologies also play the same role as libraries in software programming
tasks. These libraries of deﬁned classes and relationships can be reused thereby reducing development
time. Using a top-level ontology also enables more effective quality assurance and governance when de-
veloping ontologies. The usage of top-level ontologies for integrating information and sharing knowl-
edge among heterogeneous sources has been motivated in various related works (Baumgartner and Rets-
chitzegger, 2006). Top-level ontologies have been used in various domains including situation awareness,
pervasive systems (Stevenson et al., 2009), biomedical information systems, government and US military
system (Semy et al., 2004), emergency management (Elmhadhbi et al., 2019) and manufacturing (Otte
et al., 2018).
4.4. Basic Formal Ontology (BFO)
BFO is a top-level ontology, which has been introduced to support information retrieval, analysis and
integration in diverse domains. BFO is at the core of the Open Biological and Biomedical Ontology
(OBO) foundry1, which is an initiative aiming to develop interoperable ontologies based upon shared
principles and architecture. BFO establishes a common ground for ontology development and is the top-
level ontology used by OBO foundry ontologies.
The Industrial Ontologies Foundry (IOF)2is an initiative similar to OBO, dedicated to the development
of ontologies for industry. As part of IOF work, the choice of a relevant top-level ontology is under study.
H. Karray et al. / ROMAIN 7
Both DOLCE and BFO were used when developing proof-of-concept for IOF ontologies, as they are two
well-known top-level ontologies that are widely used in different domains (Guarino, 2017).
For ROMAIN, we consider the use of BFO as it has already been used successfully in OBO Foundry for
providing a common ground for several interoperable ontologies and it is in the process of becoming an
ISO Standard (ISO 21838-2). Additionally, BFO is sufﬁciently small and covers classes to model functions
and roles, which are necessary for engineering knowledge representation. Also, it is quality controlled
through aggressive review and reuse by a sizable community of users. ROMAIN imports portions of BFO
compliant mid-level ontologies such as Common Core Ontologies (CCO; CUBRC, 2014) and Information
Artifact Ontology (IAO; Ceusters, 2012).
Fig. 3. The structure of BFO 2.0
As shown in Figure 3, BFO 2.0 consists of two main categories of entities: continuants and occurrents.
Continuants are entities which continue to exist through time while maintaining their identity and have
no temporal parts but are associated with a history. A history names an entity belonging to the realm of
occurrents (which can have temporal parts). Occurrents are entities that occur, happen, unfold or evolve
with time. BFO:occurrent can be either an entity that unfolds itself in time, or it is the instantaneous
boundary of such an entity. Table 1 summarizes key categories encountered in BFO as deﬁned by Arp
et al. (2015).
5. Reference Ontology of Industrial Maintenance (ROMAIN)
This section provides more details about the proposed maintenance ontology in terms of objectives,
requirements, and competency questions.
5.1. ROMAIN objectives
The scope of the current version of the ontology is limited to those classes and relationships needed to
represent various entities related to the maintenance management process as described in Section 2.
5.2. ROMAIN Requirements
According to what is discussed in the previous sections, the key requirements of the ROMAIN ontology
are listed as follows:
8H. Karray et al. / ROMAIN
BFO’s classes deﬁnitions
BFO:entity Anything that exists or has existed or will exist.
BFO:continuant An entity that continues or persists through time while maintaining its identity,
and has no temporal parts. It is a dependent or independent object.
BFO:occurrent An entity that occurs, happens, unfolds or develops in time: events or processes
BFO:independent continuant A continuant entity that is the bearer of qualities. It can maintain its identity and
existence through gain and loss of parts, dispositions and/or roles, and through
changes in their qualities.
An entity that is dependent on one or more other independent continuants that
can serve as its bearer. It is similar to complex continuant patterns of the sort
created by authors or through the process of evolution.
An entity that depends on one or more speciﬁc independent continuants for its
existence. It exhibits existential dependence and has two subcategories: quality
and realizable entity.
BFO:process An occurrent entity that exists in time by occurring or happening, has temporal
parts, and always depends on at least one material entity. It can be partitioned
into temporal parts in different ways and at different levels of granularity.
BFO:quality A speciﬁcally dependent continuant that depends or inheres in an entity at all
and is fully exhibited or manifested or realized in that entity.
BFO:disposition A realizable entity whose bearer is some material entity.
BFO:function A function is a disposition that exists in virtue of the bearerâ ˘
Zs physical make-
up and this physical make-up is something the bearer possesses because it came
into being either through evolution (in the case of natural biological entities) or
through intentional design (in the case of artifacts), in order to realize processes
of a certain sort.
BFO:role A realizable entity which exists because the bearer is in some special physical,
social, or institutional set of circumstances in which the bearer does not have to
be, and is not such that, if it ceases to exist, then the physical make-up of the
bearer is thereby changed.
BFO:temporal region An occurrent entity that is part of time as deﬁned relative to some reference
–The ontology has to be aligned with a top-level ontology and reuses classes from mid-level ontolo-
–The ontology has to capture the core notions related to the domain of maintenance management.
–The ontology has to use the established terminology of the maintenance management domain.
–The ontology has to follow the design principles for ontology development.
5.3. Competency questions
Grüninger and Fox (1995) proposed competency questions (CQs) as a technique for deﬁning the ontol-
ogy speciﬁcations. These CQs consist of a set of questions, stated in natural language, that the ontology
must be able to answer. Below are some of the examples of the CQs that ROMAIN, and its derivative
application ontologies, should be able to answer:
–How many breakdown work orders were executed?
–How many replace orders were executed before the expected replacement interval <x> of operating
–What is the total cost of failures on the maintainable item <A> which should have no failures under
its scheduled restoration maintenance strategy?
–Did any scheduled restorations occur before the target interval of <x> operating hours?
–What is the cost of the work done initiated by an inspection?
H. Karray et al. / ROMAIN 9
6. ROMAIN structure
This section describes the core classes of ROMAIN. Figure 4 illustrates the structure of the ontology
using some of its main classes. Descriptions of the various classes and their relationships are given in the
sections below. We note that the preﬁx ’ROM’ is used to identify ROMAIN classes.
Fig. 4. The class diagram showing the relationships between some of the continuants, occurrents, and information entities
6.1. Relations in ROMAIN
Before introducing ROMAIN classes, we introduce the used relations in order to well understand the
deﬁnitions of these classes and their proprieties. There are two groups of relations in the proposed ontol-
ogy. The ﬁrst group is composed of the relations speciﬁc to ROMAIN and the second group is composed
of the generic relations imported from other external ontologies. The Relations Ontology (RO; Smith
et al., 2007) in particular is used extensively. RO is a collection of relations intended primarily for stan-
dardization across ontologies in OBO Foundry. RO is developed in the context of this Foundry initiative
and, therefore, it is in compliance with BFO. Table 2 summarizes all relations used in ROMAIN.
Continuants are the entities that continue or persist through time, such as objects and their functions
and qualities. There are three types of continuants in BFO, namely, generically dependent, independent,
and speciﬁcally dependent continuants that are discussed separately in the context of maintenance man-
agement in this section . The deﬁnitions of the ROMAIN’s generically dependent continuants, indepen-
dent continuants, and speciﬁcally dependent continuants are provided in Table 3, Table 4, and Table 5
6.2.1. Independent continuants
Independent continuants do not depend on other entities for their existence. Examples of independent
continuants in ROMAIN include the machines, equipment, devices, or components that become the sub-
jects of maintenance strategies and actions. They are considered to be instances of Artifact class in RO-
MAIN. An Artifact, a class imported from CCO, is an object that is designed by some agent to realize
10 H. Karray et al. / ROMAIN
Terms used for ROMAIN’s relations
is a entity A is a entity B means that A is subclass of B.
RO: bearer of a relation between an independent continuant (the bearer) and a specif-
ically dependent continuant (the dependent), in which the dependent
speciﬁcally depends on the bearer for its existence.
RO: has role a relation between an independent continuant (the bearer) and a role, in
which the role speciﬁcally depends on the bearer for its existence.
RO: has function a relation between an independent continuant (the bearer) and a func-
tion, in which the function speciﬁcally depends on the bearer for its
RO: has part a core relation that holds between a whole and its part.
RO: is about A primitive (i.e. undeﬁned) relationship between an information content
entity and some entity.
RO: prescribes For all classes T1 and T2, if T1 prescribes T2, then there is some in-
stance of T1, t1, that serves as a rule or guide to some instance of T2,
t2 (if T2 is a type of BFO:occurrent) or that serves as a model for some
instance of T2, t2 (if T2 is a type of BFO:continuant)..
ROM: has subject a relation between an occurrent and a dependent continuant in which the
dependent continuant is the reason to the appearance of the occurent.
RO: affects If p is a process and c is a continuant, then p affects c if and only if p
inﬂuences c in some manner, most often by producing a change in c.
RO: describes For all classes T1 and T2, if T1 describes T2 then there is some instance
of T1, t1 that presents the characteristics by which some instance of T2,
t2 can be recognized or visualized.
RO: has output a relation between a processual entity and a Continuant such that the
presence of the Continuant at the end of the processual entity is a nec-
essary condition for the completion of the processual entity.
RO: participates in a relation between a continuant and a process, in which the continuant
is somehow involved in the process.
RO: proceeds a relation between two occurents. Occurent x precedes occurent y if and
only if the time point at which x ends is before or equivalent to the time
point at which y starts.
RO: occurs on An instance level relation which holds between some processual entity
and some temporal region whenever the duration of the processual en-
tity is contained by the temporal region.
RO: is input of Inverse of has input, a relation between a processual entity and a Con-
tinuant such that the presence of the Continuant at the beginning of the
processual entity is a necessary condition for the start of the processual
ROM: violates A relation between a continuant and a directive ICE in which the con-
tinuant does not fulﬁll the directions expressed in the directive ICE.
certain functions. An Artifact is an asserted class that can assume different roles in maintenance scenario
and become an asset, a component, or a maintainable item as shown in Figure 5. These notions are further
deﬁned in the following sections.
Asset An Asset, as an independent continuant, is an Artifact that has potential or actual value to an
organization (ISO, 2014). Asset is treated as a deﬁned class in ROMAIN. It is formally deﬁned as an
Artifact that bears an Asset Role for an organization.
Component AComponent is an Artifact that bears a Component Role. A Component Role is a role that
is borne by an Artifact when it become a part of another artifact, to provide a particular function.
Maintainable Item AMaintainable Item is an artifact when it bears the Maintainable Item Role in the
context of a maintenance strategy. An Asset can be a Maintainable Item or it can be composed of one or
more maintainable items. Each Maintainable Item has one or more functional failures. A single Compo-
nent can also become an instance of Maintainable Item. The selection of what is or what is not a Main-
H. Karray et al. / ROMAIN 11
tainable Item is a decision of the maintenance group and a result of maintenance strategy development
process. It depends on the resources and capabilities available at a speciﬁc site. For example, they may
choose to rebuild pumps, in which case the maintainable items will be the bearings and other items that
make up the pump. Or they may choose to send the pump to an external rebuild shop in which case the
pump is the maintainable unit.
Fig. 5. Different roles an artifact can bear in the domain of maintenance
6.2.2. Speciﬁcally Dependant Continuants
A speciﬁcally dependent continuant (SDC) is an entity that depends on other independent continuants
for its existence. Quality, function, and role are sub-types of this SDC class. Functions, in particular, are
directly used when representing a functional failure in an artifact since a failure is the termination of the
ability of an artifact to perform a required function. A failure is often a consequence of some change
or degradation in one or more qualities of the artifact or its parts. A failure can be attributed to one or
more nonconformities. A nonconformity is a deviation from a requirement. It is one of the core classes in
ROMAIN and is hence described in more detail below.
Nonconformity In general, nonconformity is non-fulﬁllment of a requirement or expectation. In RO-
MAIN, Nonconformity is formally deﬁned as a speciﬁcally dependant continuant that inheres in a material
entity and deviates from a design speciﬁcation. It can be a loss of quality, function, or capability in an item
that results from a degradation process. Each nonconformity can also trigger other degradation processes
that may cause more nonconformities in the same bearer or other external bearers. For example, reduced
traction in a tire ( degraded function = functional nonconformity ) can be the result of worn tire treads. In
this example, “shallow tread" is a physical change or a degraded quality that is the basis for the functional
nonconformity in the tire and “tread depth" is a quality borne by the tire that changes during the course of
“wear process". Both “shallow tread" and “reduced traction" can be regarded as instances of nonconfor-
mity class in connection with speciﬁc design requirements. Also, it should be noted that nonconformity is
a deﬁned class since it is not an inherent characteristic of a quality or function.
6.2.3. Generically Dependant Continuants
In ROMAIN, the Information Content Entity (ICE) class, a subclass of generically dependent continu-
ant, is used for different purposes as shown in Table 3. Descriptive, designative, and directive ICE are the
three sub-types of ICE in ROMAIN that are imported from CCO. Descriptive ICEs are used to describe
various entities such as maintenance strategies, actions, work orders, and notiﬁcations. Directive ICEs are
used to describe plans and procedures, such as maintenance plan, through a set of prepositions. Mainte-
nance Work Order Record is an example of a descriptive ICE that describes a desired Maintenance Action.
It is also used to record the actual action taken and captures the costs and resources required and actually
used in the execution of the maintenance action. Figure 6 shows the main ﬁelds of a Maintenance Work
Order Record. While there are many data ﬁelds associated with a single maintenance work order record,
the ones listed in this ﬁgure are the most common ﬁelds. For example, most work order records contain
12 H. Karray et al. / ROMAIN
data ﬁelds that correspond to start date and system status code that are considered to be sub-types of date
identiﬁer and code identiﬁer, respectively. Maintenance Notiﬁcation is a descriptive ICE that describes an
Fig. 6. Data ﬁelds associated with a maintenance work order record
ROMAIN Generically dependent continuants
Common term (ROMAIN class label) Deﬁnition
Descriptive Information Content Entity (De-
scriptive Information Content Entity)
An IAO:information content entity that consists of a set of propositions
that describe some Entity.
Designative Information Content Entity (Desig-
native Information Content Entity)
An IAO:Information Content Entity that consists of a set of symbols
that denote some Entity.
Directive Information Content Entity (Directive
Information Content Entity)
An IAO:Information Content Entity that consists of a set of propositions
or images (as in the case of a blueprint) that prescribe some Entity.
Maintenance work order record (Maintenance
Work Order Speciﬁcation)
An IAO:information content entity describing a desired
Maintenance notiﬁcation (Maintenance Notiﬁ-
An IAO:information content entity describing a ROM:Non-conformity.
Maintenance plan (Maintenance Plan Speciﬁca-
An IAO:plan speciﬁcation containing Maintenance Strategies and
Maintenance Standard Work Procedures for a ROM:Maintainable Item
over its lifecycle.
Maintenance strategy speciﬁcation (Mainte-
nance Strategy Speciﬁcation)
An IOA:information content entity describing the ROM:Maintenance
Strategy Type to manage a speciﬁc functional failure.
Maintenance strategy type (Maintenance Strat-
An IAO:information content entity resulting from ROM:maintenance
strategy development process for a ROM:Maintainable Item.
Maintenance schedule (Maintenance Schedule
An IOA:information content entity describing a list of
ROM:Maintenance Work Order Speciﬁcation to be executed in a
deﬁned BFO:one-dimensional temporal region (i.e. temporal interval).
Maintenance standard work procedure (Stan-
dard Work Procedure Speciﬁcation)
An IAO:plan speciﬁcation that describes the steps required for a speciﬁc
Requirement (Requirement Speciﬁcation) An IAO:information content entity that prescribes a need or expectation
to be met
H. Karray et al. / ROMAIN 13
ROMAIN Independent continuant
Common terms (ROMAIN class descriptor) Deﬁnition
Asset (Asset) A CCO:artifact that bears an ROM:Asset Role.
Component (Component) A CCO:artifact that bears a ROM:Component Role.
Maintainable item (Maintainable Item) A CCO:artifact when it bears the ROM:Maintainable Item Role
ROMAIN Speciﬁcally dependant continuant
Common term (ROMAIN class label) Formal Deﬁnition
Component role (Component Role) A BFO:role that is borne by an CCO:artifact(s) when it becomes a part
of another BFO:material entity, to provide a particular BFO:function.
Maintainable item role (Maintainable Item
A BFO:role borne by a group of one or more CCO:artifact(s) as an
output of a ROM:Maintenance Strategy Process.
Asset role (Asset Role) A BFO:role that is borne by an CCO:artifact to deliver potential or ac-
tual value to an CCO:organization
Function (BFO:function) A BFO:function is a disposition that exists through intentional design
in order to realize processes of a certain sort.
Non-conformity (Nonconformity) A BFO:speciﬁcally dependent continuant that inheres in a material en-
tity and deviates from a design speciﬁcation.
Processes and events are the main types of occurrents in ROMAIN as shown in Table 6 and Figure 7.
Common term (ROMAIN class label Formal Deﬁnition
Degradation process (Process Of Degradation) A BFO:process that results in the loss of a desired quality or function.
CCO:Stasis (State) A BFO:Process in which some BFO:independent continuant endures
and one or more of the dependent entities it bears does not change in
kind or intensity
Functional failure (State Of Failure) A ROM:state during which a CCO:artifact is unable to perform its
Degraded state (State Of Degradation) A ROM:state during which a CCO:artifact bears an undesirable
BFO:quality or BFO:function.
Maintenance action (Maintenance Action) A BFO:process to perform work on a CCO:artifact according to a
ROM:Maintenance Work Order Speciﬁcation.
Triggering event (Triggering Event) A BFO:process resulting in an action.
Failure (Failure Event) A BFO:process that precedes the ROM:State of Failure.
Process of Degradation Artifacts can undergo a change in their speciﬁcally dependant continuants. The
Process Of Degradation is a process in which a desired quality, function, or capability is lost over time or
an undesirable quality, function, or capability is gained over time. The output of the Process Of Degrada-
tion is a Non-conformity that may or may not be observable. When a Non-conformity inheres in an artifact,
then the artifact enters the State Of Degradation. When an item is in State Of Degradation, it doesn’t
necessarily mean that the item is failed. However, degradation might lead to failure.
Failure Event In general, an event is treated as a process in BFO. A Failure Event is a process that
precedes a State Of Failure. When a Component is in State Of Failure it is unable to perform its intended
function. The relation between an event and a state is inspired here by the the point of view presented
by Galton (2012), in which a state can be initiated or terminated by an event. It should be noted that
failure event is not a temporal region to specify the moment that failure happens. Rather, it is a process
that occurs at some point in time or during a time interval. A failure event occupies a temporal region,
14 H. Karray et al. / ROMAIN
Fig. 7. The occurrents related to degradation and failure processes
either zero-dimensional or one dimensional. If the failure event is instantaneous, then it occupies a zero-
dimensional temporal region and if it takes place during an interval, then it occupies a one-dimensional
Triggering Event Triggering Event is a process. For example, it might be that a preset maintenance time
interval has been reached, resulting in initiation of a maintenance process as shown in Figure 8. This ﬁgure
also demonstrates how a ﬁxed-interval repair strategy, as a directive ICE, can prescribe a triggering event.
Axioms represents that facts that are always true in the domain. In ROMAIN, different types of class
axioms, including equivalent, subclass, and disjoint class axioms, have been used to impart more formality
to the ontology. Table 7 shows some examples of class axioms implemented in ROMAIN. The axioms are
represented using Manchester Syntax from OWL 2.0 as it is a more compact and readable representation.
H. Karray et al. / ROMAIN 15
Fig. 8. Relationships between a Maintainable item, maintenance strategy, and maintenance action
Some examples of class axioms in ROMAIN
Class: MaintainableItem EquivalentTo:
cco:Artifact AND ( hasRole SOME
Equivalent class axiom for Maintainable Item.
Class: Asset EquivalentTo: cco:Artifact
AND ( hasRole SOME Asset Role)
Equivalent class axiom for Asset.
Class: StateOfFailure SubClassOf:
(precededBy SOME ProcessOfFailure) AND
(hasParticipant SOME Artifact )
Every State of Failure is preceded by a Process of Fail-
ure and has at least one Artifact participant
Class: ProcessOfDegradation SubclassOf:
(precedes SOME StateOfDegradation) AND
(hasOutput SOME Nonconformity)
Every Process of Degradation precedes a State of
Degradation and has a Nonconformity as the output
7. Validation, Veriﬁcation and Discussion
Ontology veriﬁcation is the ontology evaluation which compares the ontology against the ontology
speciﬁcation document (ontology requirements and competency questions cf. section 5) thus ensuring
that the ontology is built correctly (Suarez-Figueroa and Gómez-Pérez, 2008). In other words, it allows
us to answer the question “are we producing the ontology right?”. Ontology validation is the ontology
evaluation that compares the meaning of the ontology deﬁnitions against the intended model of the world
aiming to conceptualize. In other words, it permits to answer the question “Are we producing the right
ontology?". Hence, in order verify and validate a reference ontology as ROMAIN, we estimate that we
need to consider a real industrial context and real data set. In the following section, we examine the
competency questions listed in Section 5.3 according to the maintenance strategy effectiveness use case
by using real-life data.
16 H. Karray et al. / ROMAIN
7.1. Use case: maintenance strategy effectiveness
Our motivating question is to determine how effective the selected maintenance strategy is in practice.
Having an effective maintenance strategy will reduce unanticipated events leading to improved uptime,
reduced costs and more effective use of maintenance resources. Common manifestations of ineffective
strategy are assets failing before their scheduled restoration or discard times and assets being replaced or
restored while they are still functional. In addition, the execution of the strategy can be ineffective. For
example if assets with on-condition tasks are failing unexpectedly then the on-condition task is obviously
not effective at detecting the deterioration. Once ineffective tasks are identiﬁed, engineers can investigate
and understand why the strategy is ineffective. Currently the process to investigate maintenance strategy
effectiveness is triggered by observations by maintenance personnel after an unexpected failure event. The
investigation, usually done by an engineer or maintenance planner, requires access to data in a number of
locations. For example, speciﬁc work order records, preventative maintenance strategy and work history
in the CMMS (all stored in different tables), failure investigations in Excel or Word ﬁles, RCM analysis
(also usually in Excel ﬁle). It can take days or even weeks to pull together the right data and perform
the analysis. A major issue is that most of the data is stored as unstructured, free text. As a result some
pre-processing of the unstructured free text using natural language processing or rule-based methods may
The data for this maintenance strategy evaluation use case comes from the CMMS used by an operator
of Heavy Mobile Equipment (HME), one of the most expensive maintainable items is the engine. We have
records of all tasks executed on the engines of four identical HME assets in their ﬁrst 18,000 operating
hours. We show an extract of these records in Figure 9. We aligned the data schema with ROMAIN
classes and then we manually instantiated this part of data via the Protégé editor in order to use it in our
In this table, columns 1, 5, 6, 7 and 8 are as-downloaded from the CMMS. Data in columns 2, 3
and 4 have been added by the engineer to assist with analysis. While the data represented may seem
straightforward to analyze, the engine is only one of many maintainable units on this asset, and operators
can own, tens to thousands of assets that might beneﬁt from maintenance strategy effectiveness analysis.
Therefore, automation of this process is an inevitable necessity.
The competency question ‘how many breakdown orders were executed?’ is straightforward and would
be usually done by an engineer generating a pre-conﬁgured report for EQ1 engine from the CMMS, add
in columns 2-4, and then use PivotTables in Excel to ﬁlter the data. This exercise then needs to be repeated
for each sub-system of interest each time the analysis needs to be done. Some engineers will use Excel
macros to help. In contrast, the SPARQL query for this ﬁrst competency question is presented in ﬁgure
10, with its associated result (the SPARQL query language can be used to express queries across diverse
data sources). Breakdown orders are triggered by failure events, which are zero-dimensional temporal
regions. Therefore, counting the number of breakdown orders comes down to counting the number of
failure events, expressed in the request in ﬁgure 10.
H. Karray et al. / ROMAIN 17
Fig. 9. All work order records associated with work on the engine of heavy mobile equipment EQ1 over its operating life to
Fig. 10. SPARQL query for ﬁrst competency question
The competency question ‘did any scheduled repairs or restoration occur before the target interval of
<x> operating hours?’ The RCM process of this maintainable item has identiﬁed a ﬁxed interval restora-
tion task at 8,000 operating hours and a ﬁxed interval replacement task at 16,000 operating hours. Assum-
ing a window of + or – 500 hours as being acceptable we can see that the ﬁrst ﬁxed interval restoration
18 H. Karray et al. / ROMAIN
task at 7,683 hours is acceptable but the second one at 15,155 operating hours was not executed. We know
it was not executed because there are no actual costs associated with the work. We also note that there is a
ﬁxed interval replacement task that occurred at 14,788 hours. There are two alarming issues here. The ﬁrst
is that the replacement work occurred at 14,788 operating hours, well short of the expected 16,000 hours.
The second issue is that there are duplicate ﬁxed interval tasks in the CMMS system and one (the ﬁxed
interval restoration task at 16,000) hours should be removed as there is a ﬁxed interval replacement also
at 16,000 hours. Obviously, the planner has ignored the ﬁxed interval restoration task generated by the
CMMS at 15,155 hours but the task is still being generated in the CMMS system and should be removed.
Since the records at 14,788 and 15,155 will be identiﬁed in the analysis, the duplicate can be removed.
The SPARQL request for this second competency question is presented in Figure 11. The results show
the identiﬁer of the work order, the identiﬁer of the event triggering the work order, and the value of
operating hours at which the work order occurred. The ﬁrst step of the request consists of querying the
events triggering scheduled work orders (ﬁrst line of the query). Then, work orders related to those events
are identiﬁed (second and third line of the query). Finally, only the work orders with less than 16,000
cumulative hours are kept in the results. The results present the identiﬁer of the work orders and the events
in the ontology, together with the cumulative hours value for each work order.
Fig. 11. SPARQL query for second competency question
The ability to write a single SPARQL query which can be run for a list of assets at different functional
locations over the existing methods is signiﬁcantly more efﬁcient and transparent to current engineering
practice. As mentioned earlier, the current approach is to have an Excel spreadsheet and pivot table for
each query. Version control is difﬁcult to manage and the number of Excel sheets quickly grows to be
unmanageable. Not to mention that the process of cutting and pasting, organizing and making pivot tables
is laborious and prone to human error.
The SPARQL queries presented in this section have been run on a version of ROMAIN containing the
work orders presented in ﬁgure 9. The results presented in ﬁgure 11 (WO16, WO5, WO14) refer to the
17th, 6th, and 15th lines of ﬁgure 9.
7.2. Evaluation of ROMAIN according to best practices and principles
In evaluating the ROMAIN ontology we consider the requirements listed in 5.2. The ontology is aligned
with a top-level ontology, the BFO. Therefore, it can be aligned and extended with classes from BFO
compliant mid-level ontologies such CCO and IAO as shown in Figure 12. This ﬁgure also shows how
BFO-aligned ontologies for activities in the maintenance management process that use processes such as
planning, scheduling and costing, can be tailored to maintenance by reference to ROMAIN concepts.
H. Karray et al. / ROMAIN 19
Fig. 12. A mapping of the relationships between BFO-compliant ontologies to enable development of an ontologies for main-
tenance management activities. [The arrow sense shows the sense of import, e.g. CCO imports BFO, and ROMAIN imports
The ROMAIN ontology captures core notions and terminology related to maintenance management as
demonstrated by the alignment to authoritative maintenance literature (Coetzee, 2004; Kelly, 2006; IEC,
2011, 2016) as well as maintenance practitioner input. Finally the ontology follows the design principles
The main challenge in the development of a BFO-conformant ontology for maintenance has been the
absence of tried, tested and accepted BFO-aligned classes and relations for the engineering domain. In
2017, an international group of volunteers started the Industrial Ontology Foundry (IOF) with the objective
of developing open and principles-based reference ontologies for the manufacturing domain. While work
on this is underway, the group has not yet reached the point of agreeing on some of the classes that are
used in the ROMAIN ontology described here. As a result, the ROMAIN authors have made judgments
based on views in the IOF community that have not yet stabilized.
One challenge has been the use of the class of maintenance action and whether this is an Information
Entity as in a description of the work to be done, or a process in the doing of the work. In ROMAIN we
place Maintenance Action in the occurrent section of the ontology but describe the work to be done in a
Maintenance Work Order Speciﬁcation as a directive information entity.
Another challenge has been related to deﬁning a condition or state of a component. In maintenance we
have the notion of a component degrading. Maintenance is deﬁned as the “actions intended to retain an
item in, or restore it to, a state in which it can perform a required function" (IEC, 2016). Hence mainte-
nance actions are dependent on the state of the component and the effect of this state its ability to perform
a required function. In ROMAIN we have deﬁned two states, a State of Failure and a State of Degradation.
These are sub-classes of process class. Transition into a State of Degradation is represented by a process
described by a Triggering Event. The triggering event class proved to be important in resolving issues
around describing this transition. The transition between a State of Failure and a State of Degradation
remains challenging as it depends on deﬁning a function and the level for that required function. For ex-
ample, one person may decide that the tire tread should be 0.4 mm deep but another that 0.8 mm deep is
necessary. A tire tread depth of 0.6 mm will be regarded as in a State of Degradation by the ﬁrst but a
State of Failure by the second. We expect dealing with transitions to be an area of fertile discussion in the
future. For ROMAIN, the main challenge has been the immature status of the application of the BFO as
a top-level ontology in engineering. While we are committed to the importance of exploiting foundation
ontologies in our work, there are challenges in BFO concerning some aspects such as the deﬁnition of
Generically Dependent Continuant, mereology and constitution. All of these challenges have been bought
to the attention of the BFO community.
20 H. Karray et al. / ROMAIN
Deﬁning the notion of nonconformity also presented a challenge when developing the ontology. Non-
conformity is the violation of both articulated and unarticulated requirements. However, representation
of unarticulated requirements is not possible as they are not speciﬁed in design speciﬁcations and tech-
nical documents. Therefore, we had to limit the formalization of the notion of nonconformity to explicit
requirements only. Additionally, nonconformity is a multifaceted entity as it can be an undesired quality
or function. Therefore, we had to directly classify it under Speciﬁcally Dependant Continuant such that
it covers different types of nonconformities. Perhaps application ontologies may need to create more spe-
ciﬁc classes of nonconformity depending on their needs. Finally, in many cases, nonconformity is real-
ized when an item lacks a function but stating ’A lacks B’ is difﬁcult due to the constraints imposed by a
realist approach. There are workarounds for this situation but it needs to be stated outside of OWL since
it involves instance-to-class relations (Ceusters et al., 2007).
The subject of causality has always been a challenging subject for ontologists and they have adopted
different approaches for handling this subject philosophically and ontologically (Galton, 2012). In the do-
main of maintenance, proper formulation and elucidation of the notion of causality is essential for effec-
tive root-cause analysis. Usually conjunction of multiple states of degradation may cause the maintainable
item to enter the state of failure. Deﬁning causality links between various states, processes, and events
need further analysis. Also, there are different types of causal relations such as affecting, enabling, and fa-
cilitating between various entities that need to be delineated formally when creating a reference ontology
Like others before us, developing a coherent set of axioms has proved challenging. These have to be
selected carefully as there are many hidden dependencies in maintenance management. These will be
more fully considered in future work.
One of the unique aspects of this project was including a maintenance expert in the development team.
As a results, the semantics of the ontology was aligned early on with the established standards and models
in the ﬁeld. One of the pitfalls of most ontology development projects is involving domain experts towards
the last stages of design process only for test and validation purposes. We used GitHub as the platform for
collaborative ontology development. The issue tracking mechanism of GitHub was particularly useful for
suggesting revisions and tracking changes. The ontology is available on github3.
This work presents the ﬁrst building blocks of ROMAIN, a BFO-compliant reference ontology for
maintenance management. We intend to expand this work further while making it open for others to de-
velop a set of ROMAIN compliant modules covering other processes and applications in maintenance.
Extension of this initial work will allow the community to move towards an expanded reference ontol-
ogy for maintenance management. It is intended to help stimulate developments in maintenance-related
domain ontologies considering using BFO as a top-level ontology (as explained in ﬁg 12). One of the
strengths of the adopted methodology for ontology development in this work was relying on real-life
maintenance data for testing and evaluating the ontology. We believe conforming to the vocabulary that is
widely used by maintenance professionals and practitioners is a major catalyst for widespread acceptance
and uptake. As the ontology evolves, some of the less generic classes of the ontology might be moved to
the application-speciﬁc extensions of the ontology, thus leaving the core classes application-agnostic.
Authors would like to thank members of the IOF (Industrial Ontologies Foundry) community for their
many useful comments.
H. Karray et al. / ROMAIN 21
Melinda Hodkiewicz thanks the BHP Fellowship for Engineering for Remote Operations and
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