(VAR X Y Z )
(RULES 
        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)))
        
)