(VAR x)
(RULES
  f(g(g(x))) -> a
  f(g(h(x))) -> b
  f(h(g(x))) -> b
  f(h(h(x))) -> c
  g(x) -> h(x)
  a -> b
  b -> c
)
(COMMENT due to A.Middeldorp, R_1 in p.14 of \cite{Oku98})