Smtlib2 parser does not parse Let terms in formulas correctly

See https://gist.github.com/edmcman/6938918#file-example1-smt2

    ed@ed-Ubu64 /tmp$ cvc3 -lang smt2 example1.smt2 
    sat
    ed@ed-Ubu64 /tmp$ z3 -smt2 example1.smt2 
    sat
    ed@ed-Ubu64 /tmp$ boolector --smt2 example1.smt2 
    sat
    ed@ed-Ubu64 /tmp$ ~/f13/stp/build/stp --SMTLIB2 example1.smt2 
    syntax error: line 7
    syntax error, unexpected LET_TOK
      token: let
    Fatal Error: 

The problem is that a term is bound inside of a formula.  https://github.com/stp/stp/pull/9 was supposed to fix this, but it was apparently incomplete.  I have spent more time looking at the smtlib2 parser, and I believe this is the correct way to fix it.