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