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

Label:
[g(f(a())) -> f(g(f(a())))] = 0
[g(f(a())) -> f(f(a()))] = 1
[f(f(a())) -> f(a())] = 0