Copyright c 1996-2011 Manuel Hermenegildo and José Francisco Morales. This document may be freely read, stored, reproduced, disseminated, translated or quoted by any means and on any medium provided the following conditions are met: 1. Every reader or user of this document acknowledges that is aware that no guarantee is given regarding its contents, on any account, and specifically concerning ... [Show full abstract] veracity, accuracy and fitness for any purpose. 2. No modification is made other than cosmetic, change of representation format, translation, correction of obvious syntactic errors, or as permitted by the clauses below. 3. Comments and other additions may be inserted, provided they clearly appear as such; translations or fragments must clearly refer to an original complete version, preferably one that is easily accessed whenever possible. 4. Translations, comments and other additions or modifications must be dated and their au-thor(s) must be identifiable (possibly via an alias). 5. This licence is preserved and applies to the whole document with modifications and additions (except for brief quotes), independently of the representation format. 6. Any reference to the "official version", "original version" or "how to obtain original versions" of the document is preserved verbatim. Any copyright notice in the document is preserved verbatim. Also, the title and author(s) of the original document should be clearly mentioned as such. 7. In the case of translations, verbatim sentences mentioned in (6.) are preserved in the language of the original document accompanied by verbatim translations to the language of the traslated document. All translations state clearly that the author is not responsible for the translated work. This license is included, at least in the language in which it is referenced in the original version. 8. Whatever the mode of storage, reproduction or dissemination, anyone able to access a digitized version of this document must be able to make a digitized copy in a format directly usable, and if possible editable, according to accepted, and publicly documented, public standards. 9. Redistributing this document to a third party requires simultaneous redistribution of this licence, without modification, and in particular without any further condition or restriction, expressed or implied, related or not to this redistribution. In particular, in case of inclusion in a database or collection, the owner or the manager of the database or the collection re-nounces any right related to this inclusion and concerning the possible uses of the document after extraction from the database or the collection, whether alone or in relation with other documents. Any incompatibility of the above clauses with legal, contractual or judiciary decisions or con-straints implies a corresponding limitation of reading, usage, or redistribution rights for this document, verbatim or modified.