(VAR L X XS Y YS )
(RULES 
        app(nil, YS) -> YS
        app(cons(X), YS) -> cons(X)
        from(X) -> cons(X)
        zWadr(nil, YS) -> nil
        zWadr(XS, nil) -> nil
        zWadr(cons(X), cons(Y)) -> cons(app(Y, cons(X)))
        prefix(L) -> cons(nil)
        
)