Issue 607

Apparently there were some issues with the substitution when using bounded symbolic variables.  Not sure why these were not caught by the lambda tests.  To address them, I had to force syntactic equality in the process of computing the free variables.  The main thing I'm not proud of is basically changing the definition of -Set to a syntactic one (by abusing ==K true).  @andreistefanescu says that is probably ok.  If anybody objects to that, I can create a -SetSyn or something similar for syntactic difference.

@andreistefanescu @grosu @dwightguth please review.