(VAR f x xs xss ys yss )
(RULES 
        app(app(map,f),nil) -> nil
        app(app(map,f),app(app(cons,x),xs)) -> app(app(cons,app(f,x)),app(app(map,f),xs))
        app(app(append,xs),nil) -> xs
        app(app(append,nil),ys) -> ys
        app(app(append,app(app(cons,x),xs)),ys) -> app(app(cons,x),app(app(append,xs),ys))
        app(app(zip,nil),yss) -> yss
        app(app(zip,xss),nil) -> xss
        app(app(zip,app(app(cons,xs),xss)),app(app(cons,ys),yss)) -> app(app(cons,app(app(append,xs),ys)),app(app(zip,xss),yss))
        app(app(combine,xs),nil) -> xs
        app(app(combine,xs),app(app(cons,ys),yss)) -> app(app(combine,app(app(zip,xs),ys)),yss)
        app(levels,app(app(node,x),xs)) -> app(app(cons,app(app(cons,x),nil)),app(app(combine,nil),app(app(map,levels),xs)))
        
)