Add a way to hide predicates from the output

[tausbn]

Sometimes, it would be nice to be able to hide predicates that are not actually interesting in the output produced from Dusa.

Take for instance the beginning character creation example:

name "Celeste".
name "Nimbus".
name "Terra".
name "Luna".

These four facts will end up being included in all solutions produced, but they're completely uninteresting (except maybe for debugging/pedagogical purposes), and it would be convenient if they could just be hidden away.

I would suggest a new declaration

#hide pred1, pred2, ..., predN.

which accepts a nonempty sequence of predicates, and has the side effect of hiding these predicates in the output. Extra bonus points if solutions are enumerated modulo these projected-away predicates (that is, if two solutions only differ in the values of some hidden predicates, then they are only counted as a single solution).

Also, for debugging purposes, there could perhaps be a "show hidden predicates" checkbox in the interface, that overrides the existing hiding and shows all predicates anyway.