Problem:
 dbl(0()) -> 0()
 dbl(s(X)) -> s(s(dbl(X)))
 dbls(nil()) -> nil()
 dbls(cons(X,Y)) -> cons(dbl(X),dbls(Y))
 sel(0(),cons(X,Y)) -> X
 sel(s(X),cons(Y,Z)) -> sel(X,Z)
 indx(nil(),X) -> nil()
 indx(cons(X,Y),Z) -> cons(sel(X,Z),indx(Y,Z))
 from(X) -> cons(X,from(s(X)))

Proof:
 Containment Processor: loop length: 1
                        terms:
                         from(X)
                        context: cons(X,[])
                        substitution:
                         X -> s(X)
  Qed