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In addition,
- scan below conjunctions in case there are equalities there too,
- ignore predicate variables and "true" axioms,
- output hypotheses in the proper order,
- explicitly remove NonTrivialRing since it now survives the filtering.

This fixes Jessie3 proving less VCs than Jessie2 on the float_sqrt example.

Only one pattern has been added to Why3 for now; Why2 knows about three
other variations (commutativity of the two multiplications).

The pattern mechanism used by the Gappa printer could possibly be
generalized and live on its own, hopefully with some kind of parser so that
one does not have to build patterns by hand.

also assure that location printing in Pretty is parenthesized correctly.