diff(plus(A,B), X, plus(DA, DB))
   <= diff(A, X, DA) and diff(B, X, DB).

diff(times(A,B), X, plus(times(A, DB), times(DA, B)))
   <= diff(A, X, DA) and diff(B, X, DB).

equal(X, X).
diff(X, X, 1).
diff(Y, X, 0) <= not equal(Y, X).
