Very well, if you insist on ignoring the broader point in favor of the specific example, then replace `length` with `is`. We can unify `X is 3 * 3`, we cannot u

Very well, if you insist on ignoring the broader point in favor of the specific example, then replace `length` with `is`. We can unify `X is 3 * 3`, we cannot unify `9 is 3 * X` (at least not in SWI-Prolog v8.4.1). Therefore, we can call the thing to the left of `is` the output and the thing to the right of `is` the input.