In coroutining mode, if Number is a free variable, the call to \/2 is delayed until this variable is instantiated.
Success: \(1, -2). \(5, Result). (gives Result = -6) \(-6, Result). (gives Result = 5) Fail: \(1, 0). Error: \(A, 6). (Error 4). \(0.0, 0.0). (Error 5). \(4 + 2, -7). (Error 24). \(1, r). (Error 24).