TRS:
 {             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)))}
 Fail