(SIG
   (a   -> o)
   (c   -> o)
   (b   -> o)
   (f   o o -> o)
   (d   -> o)
)
(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)
)