(VAR m n x x' xs xs' )
(STRATEGY INNERMOST)
(RULES 
        ack(Cons(x,xs),Nil) -> ack(xs,Cons(Nil,Nil))
        ack(Cons(x',xs'),Cons(x,xs)) -> ack(xs',ack(Cons(x',xs'),xs))
        ack(Nil,n) -> Cons(Cons(Nil,Nil),n)
        goal(m,n) -> ack(m,n)
        
)