/*
   subterm(Sub,Term) :- Sub is a subterm of the ground term Term.
*/
     subterm(Term,Term).
     subterm(Sub,Term) :- 
        compound(Term), functor(Term,F,N), subterm(N,Sub,Term).

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

%  Program 9.2    Finding subterms of a term