(VAR X X1 X2 Y Z )
(RULES 
        first(0, X) -> nil
        first(s(X), cons(Y, Z)) -> cons(Y, n__first(X, activate(Z)))
        from(X) -> cons(X, n__from(n__s(X)))
        first(X1, X2) -> n__first(X1, X2)
        from(X) -> n__from(X)
        s(X) -> n__s(X)
        activate(n__first(X1, X2)) -> first(activate(X1), activate(X2))
        activate(n__from(X)) -> from(activate(X))
        activate(n__s(X)) -> s(activate(X))
        activate(X) -> X
        
)