text: if __name__ == "__main__":
      class B(Cooperative):
          def say(self):
              print "B",
      class C(B):
          def say(self, super):
              super.say()
              print "C",
      class D(B):
          def say(self, super):
              super.say()
              print "D",
      class CD(C, D):
          def say(self, super):
              super.say()
              print '!'
      CD().say()
  # emits: B D C !
  