replace_arg_fast(C,A,VAR,CC):- duplicate_term(C,CC),setarg(A,CC,VAR),!.

replace_arg(C,A,VAR,CC):-
   C=..FARGS,
   replace_nth(FARGS,A,VAR,FARGO),!,
   CC=..FARGO.
:-moo_hide_childs(replace_arg/4).

:-moo_hide_childs(replace_nth/4).
replace_nth([],_,_,[]):- !.
replace_nth([_|ARGO],0,VAR,[VAR|ARGO]):- !.
replace_nth([T|FARGS],A,VAR,[T|FARGO]):-
    A2 is A-1,replace_nth(FARGS,A2,VAR,FARGO).