decreasing clauses for loops are now required
changed the rule in the builder as well
added replaces method for CharExp and StringExp. PExp now converts CharExp/StringExp to PSymbol
set the math type of character string to Str(N) and characters to N
fixed strings in RESOLVE and added a toString method to ProgramCharExp, ProgramStringExp and StringExp
Merge pull request #136 from yushan87/cpsc372fixes

Cpsc372fixes