[Versa] Issue 100A proposal, extension functions

[Chimezie Ogbuji]

> 
> I like this, plus it solves the issue of adding all of the functions
> like "rdf:type()" etc.

Yup.. don't need class (covered by rdf:type(member)) or value (covered
by rdf:value(res)).  Though I think rdf-list and rdf-container are
still needed to cast RDF sets/lists to versa sets and lists,
especially since we have no literal representation for sets/list, we
mostly use the list/set functions.  These two functions probably need
a section describing their use as constructors, they aren't listed in
the function section.

> 
> Should we have processor produce a warning?

I'm not sure, I guess it depends on if the processor thinks using
functions this way has an ill-effect in some way or just that the user
should be aware of it.  I can imagine using them much more often than
the forward traversal operator equivalents, so would I expect a
warning raised everytime I do? I'm thinking maybe it should be left up
to the implementation to decide.