latest merge onto master

[Chris Mungall]

I just merged some changes into master. The only change in the core modules is the addition of a builtin_class/1 to owl2_model, which unifies with owl:Thing or owl:Nothing. It's not clear if calling

class(X)

Should unify X with owl:Thing or owl:Nothing. There's a predicate is_class/1 which is true if X is a declared class or a builtin class.

[Vangelis Vassiliadis]

I believe is_class is OK. owl:Thing and owl:Nothing are implicit. I wouldn't make them explicit.
Doing so would require also need to make explicit all subClassOf(X, owl:Thing), etc. which is
too ..verbose (to say at least).

[Chris Mungall]

Sounds reasonable. I'll leave as-is then. You can use is_class/1 to test if something is declared a class or an implicit class, but class/1 will never unify with thing/nothing (unless you declare them!)