(from AG01 3.17a)
(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))
sum(plus(cons(0,x),cons(y,l))) -> pred(sum(cons(s(x),cons(y,l))))
pred(cons(s(x),nil)) -> cons(x,nil)
)