(VAR x)
(RULES
  a -> c
  b -> c
  f(a,b) -> d
  f(x,c) -> f(c,c)
  f(c,x) -> f(c,c)
  d -> f(a,c)
  d -> f(c,b)
)
(COMMENT R_2 in p.14 of \cite{Oku98})