add 'and' and MIT define form predicates (tested). also re-wrote the simple define predicate, added a utility predicate def_expr for recognizing both.

added also predicates for the 3 let expressions. although bugged.
add 'and' and MIT define form predicates (tested). also re-wrote the simple define predicate, added a utility predicate def_expr for recognizing both.

added also predicates for the 3 let expressions. although bugged. in addition also added cond predicate (tested)