| ?- [user]. factorial(0,1). factorial(N,F) :- N>0, N1 is N-1, factorial(N1,F1), F is N * F1. **Here you hit <CTRL>-<D> on the empty line** user compiled, 7 lines read - 822 bytes written, 24027 ms yes | ?- factorial(5,W). W = 120 ?