(VAR f y ys )
(RULES 
        app(app(filter,f),nil) -> nil
        app(app(filter,f),app(app(cons,y),ys)) -> app(app(app(filtersub,app(f,y)),f),app(app(cons,y),ys))
        app(app(app(filtersub,true),f),app(app(cons,y),ys)) -> app(app(cons,y),app(app(filter,f),ys))
        app(app(app(filtersub,false),f),app(app(cons,y),ys)) -> app(app(filter,f),ys)
        
)