(SIG
   (f    o -> o)
   (b    -> o)
   (a    -> o)
   (c    -> o)
   (d    -> o)
   (e    -> o)
   (e'   -> o)
)
(RULES
   f(b) -> a
   f(b) -> f(c)
   f(c) -> f(b)
   f(c) -> d
   b -> e
   c -> e'
   f(e) -> a
   f(e') -> d
)