% From The Art of Eyepl, Chapter 22 — Rewriting symbolic expressions.
simplify(add(number(0), X), X).
simplify(add(X, number(0)), X).
simplify(multiply(number(1), X), X).
simplify(multiply(X, number(1)), X).

simplify(add(A, B), add(SA, SB)) :-
  simplify(A, SA),
  simplify(B, SB).