fac_fact((N, F)) :-
   N = 0,
   F = 1.
fac_step((N, F), (N1, F0)) :-
   N #> 0,
   F #= F0*N,
   N1 #= N - 1.

fac(N,F) :-
   fac((N,F)).

fac(NF) :-
   fac_fact(NF),
   garbage_collect.
fac(NF) :-
   fac_step(NF, N1F0),
   garbage_collect,
   fac(N1F0).

% ?- fac(N, F), fac(M, F), N > 10.
% ?- F = 87178291200, fac(M, F).

fac14(N,F) :-
   S0 = (N,F),
   fac_step(S0,S1),
   fac_step(S1,S2),
   fac_step(S2,S3),
   fac_step(S3,S4),
   fac_step(S4,S5),
   fac_step(S5,S6),
   fac_step(S6,S7),
   fac_step(S7,S8),
   fac_step(S8,S9),
   fac_step(S9,S10),
   fac_step(S10,S11),
   fac_step(S11,S12),
   fac_step(S12,S13),
   fac_step(S13,S14),
   fac_fact(S14).

fac15(N,F) :-
   S0 = (N,F),
   fac_step(S0,S1),
   fac_step(S1,S2),
   fac_step(S2,S3),
   fac_step(S3,S4),
   fac_step(S4,S5),
   fac_step(S5,S6),
   fac_step(S6,S7),
   fac_step(S7,S8),
   fac_step(S8,S9),
   fac_step(S9,S10),
   fac_step(S10,S11),
   fac_step(S11,S12),
   fac_step(S12,S13),
   fac_step(S13,S14),
   fac_step(S14,S15),
   fac_fact(S15).