(VAR N X XS )
(RULES 
        from(X) -> cons(X, from(s(X)))
        head(cons(X, XS)) -> X
        2nd(cons(X, XS)) -> head(XS)
        take(0, XS) -> nil
        take(s(N), cons(X, XS)) -> cons(X, take(N, XS))
        sel(0, cons(X, XS)) -> X
        sel(s(N), cons(X, XS)) -> sel(N, XS)
        
)