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

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