Prolog factorial recursion

Question

I\'m having trouble understanding the following factorial program

fact1(0,Result) :-
    Result is 1.
fact1(N,Result) :-
    N > 0,
    N1 is N-1,
    fac         


        

Answer 1

non-tailer recursion :

 fact(0,1):-!. 
   fact(X,Y):- Z=X-1,
         fact(Z,NZ),Y=NZ*X.

tailer recursion:

fact(X,F):- X>=0,fact_aux(X,F,1).
fact_aux(0,F,F):-!.
   fact_aux(X,F,Acc):- 
       NAcc=Acc*X, NX=X-1, 
    fact_aux(NX,F,NAcc).