(SIG
   (f   o -> o)
   (a   -> o)
   (g   o o -> o)
   (b   -> o)
   (c   -> o)
   (d   -> o)
)
(RULES
   f(a) -> f(g(b,b))
   a -> g(c,c)
   c -> d
   d -> b
   b -> d
)