(VAR X Y Z )
(RULES 
        from(X) -> cons(X, from(s(X)))
        first(0, Z) -> nil
        first(s(X), cons(Y, Z)) -> cons(Y, first(X, Z))
        sel(0, cons(X, Z)) -> X
        sel(s(X), cons(Y, Z)) -> sel(X, Z)
        
)