docs: def defines(self, id, domain='messages'):
          """
          Checks if a message has a translation (it does not take into account
          the fallback mechanism).
  
          @rtype: bool
          @return: true if the message has a translation, false otherwise
          """
          assert isinstance(id, (str, unicode))
          assert isinstance(domain, (str, unicode))
  
          return id in self.messages.get(domain, {})