A programming question

Hi

To collect all terms of the form skX/{0-N} – where X is a number 1, 2, ... and the term can have any number of arguments – that occur in Term I use the call

(*)   findall(Skolem, (subterm(Skolem, Term), functor(Skolem, Functor, _Arity), atom_chars(Functor, [s,k|_])), Skolems)

where

subterm(Term, Term) :-
  nonvar(Term).

subterm(Sub, Term):-
  nonvar(Term),
  functor(Term, _F, N),
  N > 0,
  subterm(N, Sub, Term).


subterm(N, Sub, Term):-
  arg(N, Term, Arg),       
  subterm(Sub, Arg).

subterm(N, Sub, Term):-
  N>1,
  N1 is N-1,
  subterm(N1, Sub, Term).


Since the call (*) occurs in several places in my program and is used very often, I wonder about its efficiency. 

Any suggestions how to speed up the call? Is replacing functor/3 by univ a good idea? Should I replace the general subterm/2 by a specialised form that takes the form of the subterm into account? 

Regards.

   --- nef