(from AG01 3.17)
(VAR k l x y)
(RULES
app(nil, k) -> k
app(l, nil) -> l
app(cons(x,l), k) -> cons(x, app(l, k))
sum(cons(x,nil)) -> cons(x,nil)
sum(cons(x,cons(y,l))) -> sum(cons(plus(x,y),l))
sum(app(l,cons(x,cons(y,k)))) -> sum(app(l,sum(cons(x,cons(y,k)))))
plus(0,y) -> y
plus(s(x),y) -> s(plus(x,y)))