/* 
----------------------------------------------- 
   two recursive calls
----------------------------------------------- 
*/

fact(0,1). 

fact(N,F) :-  
   N>0, 
   N1 is N-1, 
   fact(N1,F1), 
   F is N * F1.

/* 
----------------------------------------------- 
   one recursive call instead of two           
----------------------------------------------- 
*/

fact3(0,F,F). 

fact3(N,A,F) :-  
    N > 0, 
    A1 is N*A, 
    N1 is N -1, 
    fact3(N1,A1,F).