NO

   1: f(b()) -> a()
   2: f(b()) -> f(c())
   3: f(c()) -> f(b())
   4: f(c()) -> d()
   5: b() -> e()
   6: c() -> e'()
   7: f(e()) -> a()
   8: f(e'()) -> d()

unjoinable pair
  a() <- -> d()