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AVerification of Binary Search
Christoph Walther and Stephan Schweitzer B
Fachgebiet Programmiermethodik
Technische Universit¨at Darmstadt
Abstract. We demonstrate the use of the XeriFun system with a veri-
fication of the Binary Search method. We present the challenges encoun-
tered when working on this problem, illustrate the operation and perfor-
mance of the system and finally discuss technical improvements as well
as subjects of further research, which are not specific to our approach.
1Introduction
We develop the XeriFun system [1],[19], a semi-automated system for the veri-
fication of statements about programs written in a functional programming lan-
guage. The motivation for this development is twofold: Since we are interested in
methods for automating reasoning tasks which usually require the creativity of
a human expert, we felt the need for having an experimental base of easy access
which we can use to evaluate new ideas of our own and also proposals known
from the literature. Such an experimental base is needed, because the value of
a method ultimately can be determined only after an integration into a running
system,asthisistheonlywaytouncover
—the competing interplay with other methods being also implemented,
—the bits and pieces necessary to make a method really work, and
—the implicit assumptions of a method which may conflict with the settings
of a real application.
The second reason for the development of XeriFun origins from our experi-
ences when teaching Formal Methods, Automated Reasoning, Semantics, Verifi-
cation and subjects there like. The motivation of the students largely increase, if
they can gather practical experiences with the principles and methods teached.
Students of Computer Science expect to see the computer to solve some problem
instead of working at their own on small problems using pencil and paper only,
thus treating the whole subject as pure theoretical exercise. Of course, powerful
systems exist and may be used, e.g. NQTHM [2], ACL2 [9], PVS [11], Isabelle
[12], VSE [5], KIV [13], HOL [3], only to mention a few, which are beyond in
their abilities and the verification and reasoning problems they can handle as
compared to a small system like XeriFun .However,theperformanceofthese
systems also comes with the price of highly elaborated logics, complicated user
B{chr.walther,schweitz}@informatik.tu-darmstadt.de
Technical Report VFR 02/02
interfaces, severe installation requirements, license fees (for some of them) etc.,
which complicates their use for teaching mainly principles within the restricted
time frame of a course (if not impossible at all). The situation greatly improves,
however, when having a small, highly portable system, with an elaborated user
interface and a simple base logic easy to grasp, which nevertheless allows the
students to perform verification case studies for problems, e.g. Sorting, Search-
ing, Basic Number Theory, Propositional Logic, Protocols, Matching, Unification
etc., already known from other courses. Therefore XeriFun has been developed
as a JAVA [4] application, which the students can run after a 1 MB download on
their home computer (whatever platform it may use) to work with the system
whenever they like to do so.
In this paper, we demonstrate the use of XeriFun with a verification of the
Binary Search method.1We illustrate the challenges encountered when working
on this problem, and - based on this analysis - discuss technical improvements
to our system and also subjects of further research, which (as we feel) are not
specific to our approach.
2BinarySearch
TheBinarySearchmethodisawell-knownintroductoryexampleincourses
about Algorithms and Data Structures [10]. Searching an array of nso-called
keys for a certain key requires O(n) steps in the worst case, but needs only
O(log n) steps if the array is ordered. This is because by the ordering property,
the domain of search can be halved after each array lookup, thus caring for the
logarithmic complexity of the method. Fig. 1 displays a version of Binary Search,
where |a|denotes the size of an array aof natural numbers with indices ranging
from 0 to |a|−1.
The correctness of this algorithm is not that obvious because of the puzzling
index calculations which provide a plentiful source of programming errors. In
order to specify the correctness of the algorithm, additional notions are needed:
If we define “key ∈a:⇔∃i∈{0, ..., |a|−1}.a[i]=key”and“ordered(a):⇔
∀i, j ∈{0, ..., |a|−1}.i≤j→a[i]≤a[j]”, then the correctness requirements
for Binary Search can be stated formally as
∀key:nat, a:array. BIN SEARCH(key, a)→key ∈a(1)
and
∀key:nat, a:array. ordered(a)∧key ∈a→BINSEARCH(key, a)(2)
where statement (1) formulates the soundness requirement and statement (2) is
the completeness requirement of the algorithm. In addition, statement
∀key:nat, a:array. BINSEARC H.steps(key, a)≤blog2|a|c (3)
1Roughly speaking, this is a “medium size” problem, more complicated than, e.g.,
the verification of quicksort or mergesort,butwithlesseffort than needed for e.g.
proving the unique prime factorization theorem,theverification of heapsort or of a
first-order matching algorithm.
2
procedure BINSEARCH(key:nat,a:array):bool <=
var i:= 0;
var j:= |a|-1;
while j>ianda[i+b(j−i)/2c]6=key do
if a[i+b(j−i)/2c]>key
then j:= i+b(j−i)/2c-1
else i:= i+b(j−i)/2c+1
fi
done ;
if j>ithen return (true)fi ;
if j=i
then return (a[i]=key)
else return (false)
fi ;
end
Fig. 1. The Binary Search Algorithm
asserts that the algorithm causes only logarithmic costs (measured in terms of the
array size), where BINSEARCH.steps(key, a) counts the number of executions
of the while-loop body in Fig. 1.
We aim to verify statements (1), (2) and (3) using the XeriFun system: Data
structures are defined in XeriFun in a constructor-selector style discipline. The
data structures structure bool <= false,true and structure nat <=0,
succ(pred:nat) (representing natural numbers) are predefinedinthesystem,
and we use linear lists over natural numbers, defined by structure list <=
empty,add(hd:nat,tl:list), to represent arrays.
Procedures and while-loops are given by (recursively defined) functional pro-
cedures. Here we use the procedure
function element(a:list, i:nat):nat <=
if a=empty
then 0
else if i=0 then hd(a) else element(tl(a),pred(i)) fi
fi
to compute the value a[i] of a (non-empty) array aat index i, and we define the
tail-recursive procedure
function find(key:nat, a:list, i:nat, j:nat):bool <=
if length(a)>j
then if j>i
then if element(a,plus(i,half(minus(j,i))))>key
then find(key,a,i,pred(plus(i,half(minus(j,i)))))
else if key>element(a,plus(i,half(minus(j,i))))
3
then find(key,a,
succ(plus(i,half(minus(j,i)))),j)
else true
fi
fi
else if j=i then key=element(a,i) else false fi
fi
else false
fi
to represent the body of the while-loopinFig.1.Finally,thefunctionalversion
of the Binary Search algorithm is given as
function binsearch(key:nat, a:list):bool <=
find(key,a,0,pred(length(a))) ,
where the definitions of the predefined procedure >and the user defined proce-
dures plus,minus,half,length,member and ordered, which are required to
define the procedure find or to formulate the correctness statements (1) and
(2), are given in Appendix 6.1 or 6.2 respectively.
3AProofSketchforbinsearch
We briefly sketch the outline of the correctness and complexity proofs before we
discuss their development with our system in Section 4.
For proving the correctness of the index calculations and also for proving the
complexity statement, some properties about the >-relation must be known and
some arithmetic groundwork has to be layed. The set of the ordering lemmas
and of the arithmetic lemmas, which were used in the whole verification, are
listed in Appendices 6.1 and 6.4.
3.1 Soundness
The soundness statement
lemma binsearch is sound <= all key:nat, a:list
if(binsearch(key,a),member(key,a),true) (4)
for binsearch,2cf. (1), immediately raises the proof-obligation
all a:list, key:nat
if(find(key,a,0,pred(length(a))),member(key,a),true) (5)
2The ternary conditional if is the only connective for writing formulas in
XeriFun.Henceif(a, b, true) is written for a→b,if (a, true, b) is written for a∨b,
if(a, b, f alse) is written for a∧betc.
4
which originates from (4) simply by the execution of the non-recursively defined
procedure binsearch. This proof-goal has a straightforward generalization [15],
viz. lemma find is sound <= all i,j,key:nat, a:list
if(find(key,a,i,j),member(key,a),true) ,(6)
which then immediately proves the soundness statement (4) by first-order infer-
ences only. For proving (6), some of the ordering and arithmetic lemmas and a
further auxiliary lemma
lemma element entails member <= all i,n:nat, a:list
if(n=element(a,i),if(length(a)>i,member(n,a),true),true) (7)
relating element with member are required.
3.2 Completeness
As in the soundness case, the completeness statement
lemma binsearch is complete <= all a:list, key:nat
if(ordered(a),if(member(key,a),binsearch(key,a),true),true) (8)
for binsearch, cf. (2), immediately raises the proof-obligation
all a:list, key:nat
if(ordered(a),
if(member(key,a),find(key,a,0,pred(length(a))),true),
true)
(9)
which originates from (8) simply by the execution of the non-recursively defined
procedure binsearch. However, this proof-goal lacks to have a straightforward
generalization. The difficulty in proving (9) is typical for a statement involving
a tail-recursive procedure stemming from the body of a loop: The variables
which vary in a procedure (or in the loop-body respectively), viz. iand jin the
present case, are replaced in the statement with non-variable terms representing
the initialization of the loop-variables, viz. 0 and pred(length(a)) here, thus
preventing an induction proof upon these variables according to the recursion
structure of the tail-recursive procedure, viz. find.
Often,aremedytosuchaverification problem is to formulate an auxiliary
lemma based on a (useful) loop-invariant:Leta[i..j]denotethepartition of a
wrt. the array-indices i≤j,i.e.a[i..j] denotes the sub-array of awith bounds i
and j.Then“key ∈a↔key ∈a[i..j]” is an invariant of the while-loop of Fig.
1 leading to the auxiliary lemma
lemma in.partition entails find <= all key,i,j:nat, a:list
if(length(a)>j,
if(ordered(a),
if(in.partition(key,a,i,j),find(key,a,i,j),true),
true),
true)
(10)
5
where in.partition is defined by
function in.partition(key:nat, a:list, i:nat, j:nat):bool <=
if a=empty
then false
else if i=0
then if key=hd(a)
then true
else if j=0
then false
else in.partition(key,tl(a),i,pred(j))
fi
fi
else if j=0
then false
else if pred(i)>pred(j)
then false
else in.partition(key,tl(a),pred(i),pred(j))
fi
fi
fi
fi
Using lemma (10) and a completeness statement about in.partition,viz.
lemma member entails in.partition <= all key:nat, a:list
if(member(key,a),
in.partition(key,a,0,pred(length(a))),
true)
(11)
the completeness statement (8) about binsearch canbeprovedbyfirst-order
reasoning only. However, the proof of statement (10) requires a bunch of further
lemmas, viz.
∀key, i,j :nat, a:list. j ≤i∧key ∈a[i..j]→i=j(12)
∀key, i,j :nat, a:list. j ≤i∧key ∈a[i..j]→a[i]=key (13)
∀key, i, j:nat, a:list. key ∈a[i..j]→key =a[i]∨key ∈a[i+1..j](14)
∀key, i,j :nat, a:list. key ∈a[i..j]→key ∈a[i..j −1] ∨key =a[j](15)
∀key, i,j, h:nat, a:list.
i≤h≤j∧key ∈a[i..j]→key ∈a[i..h]∨key ∈a[h..j](16)
∀key, i,j, h:nat, a:list. ordered(a)∧key ∈a[i..j]∧a[h]>key→h>i (17)
∀key, h:nat, a:list. ordered(a)∧key > a [h]∧|a|>h→key > hd(a)(18)
6
∀key, i, j,h:nat, a:list.
ordered(a)∧key ∈a[i..j]∧key > a [h]∧|a|>h→j>h (19)
and also some of the ordering and arithmetic lemmas.3
3.3 Complexity
For measuring the costs of binsearch,thecostsof find have to be determined
first. To this effect, the number of recursive calls in find (corresponding to the
number of executions of the while-loop body in Fig. 1) is counted by the proce-
dure find.steps andthenthecostsofbinsearch are defined by the procedure
binsearch.steps:
function find.steps(key:nat, a:list, i:nat, j:nat):nat <=
if length(a)>j
then if j>i
then if element(a,plus(i,half(minus(j,i))))>key
then succ(find.steps(key,a,
i,pred(plus(i,half(minus(j,i))))))
else if key>element(a,plus(i,half(minus(j,i))))
then succ(find.steps(key,a,
succ(plus(i,
half(minus(j,i)))),j))
else 0
fi
fi
else 0
fi
else 0
fi
function binsearch.steps(key:nat, a:list):nat <=
find.steps(key,a,0,pred(length(a)))
Using binsearch.steps, the complexity statement now is formulated as
lemma binsearch log bounded <= all a:list, key:nat
if(binsearch.steps(key,a)>log2(length(a)),false,true) (20)
3It is interesting to see that the lemmas (12) - (15) refer to the bounds of the array,
known as a source of frequent programming errors. Lemma (16) states that each legal
index hseparates a partition into a pair of covering sub-partitions, and lemmas (17)
- (19) relate the ordering of array elements with the ordering of the array indices.
7
where the definition of log2 (computing the binary logarithm truncated down-
wards) is given in Appendix 6.2.4Also here, the system replaces the call of
binsearch.steps raising after some simplification steps the proof-obligation
all a:list, key:nat
if(find.steps(key,a,0,length(tl(a)))>log2(succ(length(tl(a))))
false,
true) .
(21)
This subgoal now is straightforwardly generalized to a corresponding lemma
about find.steps,viz.
lemma find.steps log bounded <= all i,j,key:nat, a:list
if(find.steps(key,a,i,j)>log2(succ(minus(j,i))),
false,
true) ,
(22)
then proving statement (20) by first-order reasoning only. However, the proof of
statement (22) requires induction and two additional lemmas, viz.
lemma complexity lemma#1 <= all i,j:nat
if(log2(succ(minus(pred(plus(i,half(minus(j,i)))),i)))
>pred(log2(succ(minus(j,i)))),
false,
true)
(23)
and
lemma complexity lemma#2 <= all i,j:nat
if(log2(succ(minus(pred(j),plus(i,half(minus(j,i))))))
>pred(log2(succ(minus(j,i)))),
false,
true)
(24)
and also some of the ordering and arithmetic lemmas.
4 Verifying binsearch with XeriFun
We now report on the efforts required to guide XeriFun to the verification
of Binary Search. First, we briefly illustrate how to use the system before we
consider the actual case. We then analyze the system’s behaviour and finally
discuss problems which either are specific to our system or constitute a general
challenge for computer supported verification.
4We use if(x>y, f alse, true),i.e. x≯y, instead of the more readable x≤ybecause
>and several lemmas about >(like transitivity, irreflexivity etc.) are predefined in
XeriFun, cf. Appendix 6.1, thus saving the additional definition of ≤and formulation
and proving of lemmas about ≤.
8
4.1 About XeriFun
XeriFun is a semi-automated system for the verification of statements about
programs written in a functional programming language, cf. [1],[19]. In a typical
session with the system, a user
—defines a (functional) program by stipulating the data structures and the
procedures of the program using XeriFun ’s language editor,
—defines statements about the data structures and procedures of the program
using XeriFun ’s language editor,
—verifies these statements and the termination of the procedures using
XeriFun ’s proof editor.
XeriFun consists of several fully-automated routines for theorem proving
and for the formation of hypotheses to support verification. It is designed as
an interactive system, where, however, the automated routines substitute the
human expert in striving for a proof until they fail. In such a case, the user may
step in to guide the system for a continuation of the proof.
When called to prove a statement, the system computes a proof-tree.An
interaction, which may be required when the construction of the proof-tree gets
stuck, is to instruct the system to prune some unwanted branches of the proof-
tree (if necessary), and then
—to perform a case analysis,
—to use an instance of a lemma or an induction hypothesis,
—to unfold a procedure call,
—to apply an equation,
—touseaninductionaxiom,or
—toinsert,tomoveortodeletesomehypotheses.
5
In addition, it may be necessary to formulate (and to prove) an auxiliary
lemma (sometimes after providing a new definition) in order to continue with
the actual proof task.
For proving the termination of a recursively defined procedure, it may be
required to tell the system a useful termination function.Usingthishint,the
system computes termination hypotheses for the procedure which then must
be verified like any other given statement. In a user guided termination proof,
the termination hypotheses are based on the predefined procedure >(which is
“assumed” to compute a well-founded relation). In order to ease such termination
proofs, the system holds a set of predefined lemmas about >, cf. Appendix 6.1.
5In XeriFun, the nodes of a proof-tree consist of sequents, and hypotheses can be
inserted into the antecedent of a sequent, can be deleted from the antecedent or
moved to the succedent. The insertion of a hypothesis implements a case analysis,
and the deletion of a hypothesis corresponds to a generalization step, cf. [15]. The
move of a hypothesis preserves equivalence and is sometimes needed (for technical
reasons) to enable a subsequent induction.
9
It is therefore useful (albeit not required) not to use other “usual” orderings
on natural numbers for a verification problem, e.g. <or ≥,thussavingthe
formulation and proving of lemmas about these orderings. This also motivates
why our formulation of Binary Search is based on >.
Having proved the termination of a functional procedure, the system may
generate additional (terminating) procedures and (verified) lemmas about them.
These system-generated procedures and lemmas are used by XeriFun ’s auto-
mated termination analysis [16], but are also useful for proofs not related to
termination. Appendix 6.3 lists the system-generated procedures and lemmas
which were actually used in the verification of Binary Search.
For proving the correctness and complexity statements for Binary Search,
some lemmas about the arithmetic functions are obviously also needed. In partic-
ular, monotonicity and estimation statements are required to justify the sound-
ness of the index calculations and to prove the loop-invariant. Here we used
XeriFun ’s Import-feature which allows to import lemmas together with their
proofs from a file. While working with the system on several verification prob-
lems, we set up certain libraries,e.g.forArithmetic and for Linear Lists,which
we use when we start to work on a new problem. Usually we begin the work
with an import from the libraries of all lemmas stating properties about the
procedures in our verification problem. This guarantees that all proven lemmas
are available when needed, where XeriFun’s lemma-filter takes care that system
performance is not spoiled by a vast amount of irrelevant lemmas. A library is up-
dated by those lemmas (not too specifictoacertainverification problem) which
were needed in the actual verification, but are not member of a library. For the
verification of Binary Search, we started with an import from our Arithmetic-
library of all lemmas about plus,minus,half and log2. Appendix 6.4 lists the
arithmetic lemmas which were actually needed when developing the proofs for
Binary Search.
4.2 Termination
XeriFun demands that the termination of each procedure which is called in a
statement is verified before a proof of the statement can be started. Therefore,
the system’s automated termination analysis [16] is activated immediately af-
ter the definition of a recursively defined procedure. The termination analysis
recognizes termination based on (nested) structural recursion in particular, as
e.g. for minus and half, which we consider as trivial termination problems. In
the Binary Search example, only log2,find and find.steps cause non-trivial
termination problems. The system proves the termination of log2, but fails to
succeed for find and for find.steps. Obviously, minus(j,i) is a termination
function for find, and providing this hint, the system generates the termina-
tion hypotheses (25) and (26), but fails to verify each of them. We therefore
demand to ignore all hypotheses except j>iin (25) and in (26), causing the
system to compute the generalized termination hypotheses (27) and (28), which
both have an automatic (induction) proof using some of the predefined, system-
10
all a:list, key,i,j:nat
if(length(a)>j,
if(element(a,plus(i,half(minus(j,i))))>key,
if(j>i,
minus(j,i)>minus(pred(plus(i,half(minus(j,i)))),i),
true),
true),
true)
(25)
all a:list, key,i,j:nat
if(length(a)>j,
if(key>element(a,plus(i,half(minus(j,i)))),
if(element(a,plus(i,half(minus(j,i))))>key,
true,
if(j>i,
minus(j,i)>minus(pred(j),plus(i,half(minus(j,i)))),
true)),
true),
true)
(26)
all i,j:nat
if(j>i,
minus(j,i)>minus(pred(plus(i,half(minus(j,i)))),i),
true)
(27)
all i,j:nat
if(j>i,
minus(j,i)>minus(pred(j),plus(i,half(minus(j,i)))),
true)
(28)
generated and arithmetic lemmas.6As find and find.steps share the same
recursion structure, we use XeriFun ’s Create Lem ma -command to make (27)
and (28) available for subsequent use. Now the verification of find.steps’ter-
mination only requires to provide the termination function minus(j,i),leaving
the remaining work to the system.
4.3 Soundness, Completeness and Complexity
Proving the soundness of binsearch (4) does not require much effort: The sys-
tem immediately comes up with the proof-goal (5), which motivates us to for-
mulate lemma (6). However, the proof of this lemma also gets stuck, but it is
immediately obvious from the failure that another lemma, viz. (7), (having a
6The motivation for these generalizations is not only to support the proofs, but to
identify the hypotheses irrelevant for the procedure’s termination. As the system gen-
erates induction axioms from the body of the terminating procedures, disregarding
irrelevant hypotheses results in induction axioms stronger than would be obtained
otherwise, see [15] for details.
11
straightforward induction proof) is needed. Given the proof of (7), (6) and in
turn (4) are easily proved.
The completeness statement is considerably harder to verify: The proof of
(8) gets stuck with the proof-obligation (9), and the task here is to develop a
lemma useful for proving this subgoal. However, different from the soundness
(and complexity) case, the required lemma cannot be formulated with the given
notions. Instead a new concept, viz. partition, has to be invented which allows to
demand the success of find(key, a, i,j )onlyforkeyskey satisfying key ∈a[i..j].
Generally, the challenge here is to spot the missing concept, of course, and,
once it is found, to represent it in such a way that subsequent proofs are sup-
ported. The problem is, however, that (except in trivial cases) it seems hard
to distinguish a good representation from a bad one a priori. We were trapped
by this problem in the following way: It seemed straightforward to us to define
a procedure function partition(a:list,i:nat,j:nat):list <= ... com-
puting the sublist of list abetween the indices iand j, and then to prove the
lemma
lemma member partition entails find <=
all key,i,j:nat, a:list
if(length(a)>j,
if(ordered(a),
if(member(key,partition(a,i,j)),find(key,a,i,j),true),
true),
true) .
(29)
However, we failed in proving (29). New subgoals were created involving the
creation of new lemmas, which in turn raised new subgoals etc., until we gave up.
An analysis of this tedious and frustrating effort revealed that the problem is the
use of partition. Roughly speaking, this procedure (using tl) strips of the list
elements in abetween positions 0 and i−1fromthebeginningofaand cuts of
(using a procedure butlast)thelistelementsbetween|a|−1andj+1 working
from the end of atowards the beginning. The problem which comes with this
approach now is that by the presence of butlast, the use of induction hypotheses
and lemmas are spoiled so that the proofs do not get through. This problem
immediately disappears if we use key ∈a[i..j], cf. function in.partition,
instead of a[i..j] as the new concept to express the lemma required to continue
with the proof of (8).
Using in.partition, lemma (10) can be formulated and a proof attempt
can be started. Now the situation develops similar to the soundness case: Proofs
get stuck, the system’s outcome is analyzed, a new lemma is spotted as the
result of the analysis, a proof of the new lemma is started which either requires
further lemmas (and so on ...) or gets through so that the proof of the statement
calling for the proved lemma can be resumed.7In course of the proof for (10),
7The way proofs are developed with the system does not differ in principle from
the way proofs are developed using pencil and paper. It is good practice to continue
with the proof obligation which is most in doubt: If it seems obvious that the spotted
12
the lemmas (12),...,(19) were invented and proved, were (16) is the central one.
Each of these lemmas evolved with less imagination from the analysis of the
systems outcome when it gets stuck in proving another lemma. The proofs use
some of the predefined and system-generated lemmas, and the proof of (10) also
uses some of the arithmetic lemmas. Finally, the completeness statement (8) is
proved by some first-order reasoning steps using (10) and (11).
Also the proof of the complexity statement requires more work than the
soundness proof, but considerably less effort than needed in the completeness
case. The proof of (20) gets stuck with the proof-obligation (21), and the missing
lemma, viz. (22), is easy to spot. Again, the induction proof of (22) gets stuck and
it is immediately obvious from the result and the induction hypotheses that the
lemmas (23) and (24) will do the job. Although these lemmas look considerably
nasty and one may expect a lot of tedious proof steps, to our surprise both
lemmas had a straightforward induction proof (using some of the predefined and
arithmetic lemmas).8After (23) and (24) being proved, the system immediately
comes up with a proof of (22), and then the proof of (20) is routine.
4.4 Analysis
The verification of Binary Search is characteristic for the way proofs are de-
veloped with XeriFun (which, as we suspect, does not differ in principle from
the way when using other systems). User interactions are required from time to
time to recover from the weakness of the first-order theorem prover, which, in
particular, is responsible for proving the base and step cases of an induction.
Such a theorem prover needs to terminate, because the system frequently deals
with proof-obligations which are not valid but can be proved by induction only.
Consequently, a sound theorem prover must be incomplete. E.g., the proof of
(6) gets stuck with a proof-obligation which is true but invalid, and therefore a
lemma, viz. (7), must be formulated and proved to finish the proof of (6).
In addition, a reasonable compromise must be made between theorem-proving
performance and system efficiency (measured both in memory and computing
time), thus providing a further source of incompleteness. To yield an acceptable
answer time, XeriFun uses several kinds of resource restrictions.Whensearch-
ing for a proof, the system’s first-order theorem prover considers verified lemmas
(and induction hypotheses). To test the applicability of such a lemma, the prover
calls itself recursively, however with additional restricted resources not to waste
too much time for deciding whether a lemma supports the proof under consid-
eration. Consequently, the prover sometimes overlooks a useful lemma and then
must be told to use it. E.g. when proving (6), the system must be called to use
lemma will do the job, one should continue first with proving the lemma. On the
other hand, if the lemma’s proof seems easy but its use is in doubt, it is advantageous
to verify first that the lemma is useful indeed. XeriFun supports both ways of proof
development at the user’s wish.
8In fact, it costs far more time T
E
Xing one of the formulas than the system needs to
prove the log2-boundedness of binsearch.
13
Sound Complete Complexity Arithmetic Termination Σ
Proof-Obligations 311 413 11 42
Insert Lemma 210 3 1 4 20
Insert Function −1− − − 1
Proof-Tree Edits 1 3 3 3 4 14
Case Analysis
Use Lemma
Unfold Procedure
−
1
−
−
3
−
−
1
2
−
−
1
−
−
−
−
5
3
Apply Equation − − − − − −
Induction − − − − − −
Hypotheses − − − 2 4 6
none 2 8 1 11 931
Tab l e 1 . User Interactions for Binary Search
(7) albeit available, because it fails to prove “j>i→j>i+b(j−i)/2c”when
operating under additional restrictions, but easily proves this subgoal when in-
structed to use (7), because then theorem proving proceeds with the resources
initially given to the system.
Table 1 illustrates the efforts when verifying Binary Search with XeriFun .9
The row Proof-Obligations displays the overall number of statements proved in
this example, separated into the subtasks “Soundness”, “Completeness”, “Com-
plexity”, “Arithmetic” and “Termination”. E.g., 11 lemmas had to be proved
for verifying the completeness statement. The 13 lemmas about the arithmetic
functions (collected in Appendix 6.4), which were used for Binary Search, are
listed separately in the Arithmetic-column. The whole case study uses 11 recur-
sively defined procedures (including those for arithmetic, cf. Appendix 6.2) for
which the system also had to prove termination.
Below, the Insert Lemma - row displays the number of lemmas we had to
create to support the proof of each of the main statements respectively, e.g. 10
for Completeness,andtheInsert Function - row shows the number of additional
definitions required for formulating these lemmas, e.g. 1 for Completeness.In
the Termination-column, the number of termination functions which we had to
submit to the system are counted in addition. The Insert Lemma -rowgivesan
account of the system’s unability to spot the “right” lemma by itself, which is
rather large for XeriFun as (except for the termination analysis) no methods
for lemma speculation (and generalization, in particular) are implemented in the
used system version 2.5.6.
The Proof-Tree Edits - row counts the number of proof-tree edits required
to guide the system to success even if the required lemmas are available, subse-
quently separated into the different activities. E.g., 3 user interventions were re-
9Predefined and system-generated procedures and lemmas are not considered in this
table as they are given “for free”.
14
quired for the completeness case.10 The Hypotheses-rules had to be called 4 times
for Termination to provide the generalizations of the termination hypotheses for
find, and had been called 2 times for Arithmetic, because the commutativity of
plus was proved by 2 nested inductions. Finally, the last row gives the number
of proof-obligations which went through the system with no user guidance at all.
5 Concluding Remarks
Thenumberofrequireduserinteractions measured in terms of additional def-
initions and proof-tree edits gives a fair account of a system’s automatization
degree. Since approx. 1/3 proof-tree edit is required for each proof-obligation
in this case study, the first-order prover shows a good performance here. This
judgement is based on a further analysis of the proofs computed by the system,
cf. [1]. For instance, after being told to use (16) in the proof of (10), the system
continued (and succeeded) without any further intervention computing a proof
of 127 steps using the 2 induction hypotheses and 20 different instances of 13
lemmas. We consider this degree of automatization as an important feature, be-
cause it relieves a user to reason which lemmas to consider and how to apply
them, which is not obvious for a large lemma set and a long proof. Also the
heuristic for chosing induction axioms and the implemented equality reasoning
behaves well here, as the system had not to be called to use a certain induction
or to apply a certain equation.11
Nevertheless, we intend to improve some of the values in Table 1: The need
for the insertion of a lemma can be decreased by extending the system with some
state-of-the-art technology for lemma speculation,e.g.[6],[7],[8],[14],[17],[18], and
- based on experiences gained from further case studies - the need of proof-tree
edits can be decreased by a finer tuning of the theorem prover wrt. the perfor-
mance vs. efficiency tradeoff. Also the steady increase of hardware performance
may improve system performance: XeriFun’s resource restrictions are controlled
by certain resource parameters. This eases to benefit from an increase of hard-
ware performance, as only the resource parameters need to be adjusted when
moving to a more powerful platform.12
However, a problem which seems far more serious is the problem of concept
formation, i.e. the invention of new concepts which are required to formulate
10 The Use Lemma row also counts the use of induction hypotheses. When distinguish-
ing proof-tree edits between Case Analysis,Use Lemma and Unfold Procedure,one
should bear in mind that very often the desired effect can be obtained by either of
the proof-rules. For instance, a cases analysis may enable the system to recognize a
useful lemma or to unfold a procedure call, a required case analysis may implicitly
evolve from the use of a lemma, etc.
11 XeriFun’s equality reasoning is based on conditional term rewriting, where the ori-
entation of the equations are computed by the system.
12 Since the right tuning of the resource parameters require a deep insight into the
system’s architecture and operation, these parameters are not at a users disposal,
but are fixed for each system version.
15
a necessary lemma. E.g. in the Binary Search verification, some new concept,
viz. in.partition, is required to express the success of find,cf.(10).The
invention of new, usefully represented, concepts require much more creativity
than needed to formulate a lemma with the notions given, or to tell the prover
what to do next. We believe that this constitutes the main obstacle for a wider
use of theorem-proving based verification systems, and major improvements in
this situation will only come with some computer support also for this problem.
6 Appendix
6.1 Predefined Procedures and Lemmas
function >(x:nat, y:nat):bool <=
if x=0 then false else if y=0 then true else pred(x)>pred(y) fi fi
∀x,y:nat. x =y→x≯y∀x,y:nat. x =y→xˆ
−1≯y
∀x,y,z:nat. x > y ∧y>z→x>z ∀x,y:nat. x > y →x6=0
∀x,y,z:nat. x ≯y∧y≯z→x≯z∀x,y:nat. x > y →y≯x
∀x,y:nat. x > y ∨y>x∨x=y∀x,y:nat. x ˆ
−1>y→x6=0
∀x,y:nat. x 6=0∧x=y→x>yˆ
−1∀x,y:nat. x 6=0∧y=0→x>y
∀x,y:nat. x > y →x>yˆ
−1∀x,y:nat. x ˆ
−1>yˆ
−1→x>y
∀x,y:nat. x > y ˆ
−1→(x>y∨x=y)13
6.2 Auxiliary Procedures
function minus(x:nat, y:nat):nat <=
if x=0
then 0
else if y=0 then x else minus(pred(x),pred(y)) fi
fi
function half(x:nat):nat <=
if x=0
then 0
else if pred(x)=0 then 0 else succ(half(pred(pred(x)))) fi
fi
function log2(x:nat):nat <=
if x=0
then 0
else if pred(x)=0
then 0
else succ(log2(succ(half(pred(pred(x))))))
fi
fi
13 We write 1 ˆ
+ for the successor function and ˆ
−1 for the predecessor.
16
function plus(x:nat, y:nat):nat <=
if x=0 then y else succ(plus(pred(x),y)) fi
function member(n:nat, k:list):bool <=
if k=empty
then false
else if n=hd(k) then true else member(n,tl(k)) fi
fi
function length(k:list):nat <=
if k=empty then 0 else succ(length(tl(k))) fi
function ordered(k:list):bool <=
if k=empty
then true
else if tl(k)=empty
then true
else if hd(k)>hd(tl(k)) then false else ordered(tl(k)) fi
fi
fi
6.3 System-Generated Procedures and Lemmas
function minus$1(x:nat, y:nat):bool <=
if x=0
then false
else if y=0 then false else true fi
fi
function half$1(x:nat):bool <=
if x=0 then false else true fi
∀x,y:nat. x ≯(x−y)→(x−y)=x∀x:nat. x ≯bx/2c→bx/2c=x
∀x,y:nat. ¬minus$1(x,y)→(x−y)=x∀x:nat. ¬half $1(x)→bx/2c=x
∀x,y:nat. minus$1(x,y)→x>(x−y)∀x:nat. half $1(x)→x>bx/2c
∀x,y:nat. (x−y)≯x∀x:nat. bx/2c≯x
6.4 Arithmetic Lemmas
∀x1,x2:nat. x1=x2→x1−x2=0 ∀x,y,z:nat. x > y →x>y−z
∀x,y:nat. x 6=0→x+y>x
ˆ
−1∀x,y,z:nat. (x+y)+z=x+(y+z)
∀x,y:nat. x +y=y+x∀x,y:nat. x ≯y→blog2(x)c≯blog2(y)c
∀x,y:nat. x ≯y→bx/2c≯by/2c∀x,y:nat. x > y →y+b(x−y)/2c≯xˆ
−1
∀x:nat. bx/2c≯xˆ
−1∀x,y:nat. (x−y)ˆ
−1=¡xˆ
−1¢−y
∀x:nat. bx/2cˆ
−1≯¥¡xˆ
−1¢/2¦∀x:nat. ¡xˆ
−1¢−bx/2c≯¥¡xˆ
−1¢/2¦
∀x,y:nat. x ≯y↔x−y=0
17
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18
