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