---- CR(R)
---- [van Oostrom, RTA 2008].
R:
f(x) -> g(f(x))
h(x) -> p(h(x))
f(x) -> h(f(x))
g(x) -> p(p(h(x)))

Label:
[f(x) -> g(f(x))] = 0
[h(x) -> p(h(x))] = 0
[f(x) -> h(f(x))] = 4
[g(x) -> p(p(h(x)))] = 4