For problem 2 in the book, your code should be able to handle queries such as:
	(infer? (length (cons 1 (cons 2 nil)) Ans) (print Ans))
and have them work.  You don't have to able to run predicates involving
arithmetic "backwards".

Note on diffaddtoend - it is *NOT* the same as diffappend.  Example:

(infer? (diffaddtoend 3 (diff X X) L)) 
 makes L = (diff (cons 3 X) X), while
(infer? (diffappend 3 (diff X X) L))
 can't be satisfied, as its first "argument" isn't a list.

Note on diffreverse - you don't have to run it backwards.