code: def add(self, method_class, action, version=None):
          
          by_version = self._by_action.setdefault(action, {})
          if version in by_version:
              raise RuntimeError("A method was already registered for action"
                                 " %s in version %s" % (action, version))
          by_version[version] = method_class
docs: Add a method class to the regitry.
  
          @param method_class: The method class to add
          @param action: The action that the method class can handle
          @param version: The version that the method class can handle