In coroutining mode, if Number is a free variable, the call to rational/2 is delayed until this variable is instantiated.
Success: rational(25, 25_1). rational(1.5, 3_2). rational(3_4,3_4). rational(9_12,3_4). rational(-6, Result). (gives Result = -6_1) Fail: rational(1, 2_1). Error: rational(A, 1_3). (Error 4). rational(3, 3). (Error 5). rational(1, r). (Error 24). rational(4 + 2, 6_1). (Error 24).