Implement assertions on the inner structure of atoms

[teiesti]

We could implement assertions on the inner structure of atoms. E.g.,

test = Assert(All(), IsFunctional("a/2"))

would ensure that a/2 is functional binary relation in every stable model. As a matter of fact, there are numerous established definitions on binary relations alone and we could extend this idea to other concepts, too.

Credit for this idea goes to Yuliya Lierler who suggested it to me via e-mail.