Solution

complex_term(X) :- nonvar(X),
                   not(atom(X)),
                   not(number(X)).
    

Another, maybe mode elegant way to define complex terms, would be to say that complex terms have an arity which is greater than 0. For this definition you would need the predicate functor/3 (see above) and predicates for comparing arithmetic expressions. We will see on day 3 how to do arithmetic in Prolog.

Back to the exercise.