Pick the goal of a theorem in a smarter way

[leotrs]

Currently the first claim found in the body of a theorem is chosen as the goal. We need to do better. First, if the theorem contains a ASSUME/PROVE clause, then the claim following PROVE should be chosen as the goal. Other than that, a good heuristic is to choose the last claim, not the first.

What to do with theorems that make multiple statements?

[leotrs]

Remember an ordinary claim is equivalent to an ASSUME/PROVE clause with an empty ASSUME part.