---- CR(R)
---- [van Oostrom, RTA 2008].
R:
-(0(),0()) -> 0()
-(s(x),0()) -> s(x)
-(x,s(y)) -> -(d(x),y)
d(s(x)) -> x
-(s(x),s(y)) -> -(x,y)
-(d(x),y) -> -(x,s(y))

Label:
[-(0(),0()) -> 0()] = 0
[-(s(x),0()) -> s(x)] = 0
[-(x,s(y)) -> -(d(x),y)] = 1
[d(s(x)) -> x] = 0
[-(s(x),s(y)) -> -(x,y)] = 0
[-(d(x),y) -> -(x,s(y))] = 0