:-arithmetic_function(!/1).
:-arithmetic_function(!/2).
:-arithmetic_function('!!'/1).
:-arithmetic_function('!!'/2).


:-op(300,xf,!).
:-op(300,xf,'!!').

!(0,1) :- !.
!(1,1) :- !.
!(N,W):-
N > 0,
N1 is N-1,
!(N1,W1),
W is N*W1.

'!!'(1,1) :- !.
'!!'(N,W):-
    N > 0,
    N1 is N-2,
    '!!'(N1,W1),
    W is N*W1.