ppError (TypeMismatch t1 t2) = "Got " ++ ppTerm t2 ++ " when expecting " ++ ppTerm t1
ppError (TypeMismatch t1 t2) = "Got " ++ ppTerm t2 ++ " when expecting " ++ ppTerm t1
ppError (TypeMismatch t1 t2) = "Got " ++ ppTerm t2 ++ " when expecting " ++ ppTerm t1