In coroutining mode, if Number is a free variable, the call to numerator/2 is delayed until this variable is instantiated.
Success: numerator(3_4, 3). numerator(9_12, 3). numerator(-3_4, -3). numerator(25, 25). Fail: numerator(3_4, 4). Error: numerator(A, 3). (Error 4). numerator(3.1, 3). (Error 5). numerator(3_4, 3_1). (Error 5). numerator(3_4, r). (Error 24). numerator(1_3 + 3_4, 13). (Error 24).