Two ideas to make it easier to express facts in the assumptions

We should ultimately pick just one of these. The current ideas are to make
Predicate subclass from Expr, so that you can express things like
Q.positive**Q.positive.  Since this is not desirable, the other idea is to
register special symbols associated with predicates in another class, QS,
automatically, so that you just do QS.positive**QS.positive.
Add missing import
Use CamelCase for class name