(VAR X Y Z )
(RULES 
        from(X) -> cons(X, n__from(s(X)))
        sel(0, cons(X, Y)) -> X
        sel(s(X), cons(Y, Z)) -> sel(X, activate(Z))
        from(X) -> n__from(X)
        activate(n__from(X)) -> from(X)
        activate(X) -> X
        
)