docs: def extend(self, *blues, memo=None):
          """
          Extends deferred operations calling each operation of given Blueprints.
  
          :param blues:
              Blueprints or Dispatchers to extend deferred operations.
          :type blues: Blueprint | schedula.dispatcher.Dispatcher
  
          :param memo:
              A dictionary to cache Blueprints.
          :type memo: dict[T,Blueprint]
  
          :return:
              Self.
          :rtype: Blueprint
  
          **--------------------------------------------------------------------**
  
          Example::
  
              >>> import schedula as sh
              >>> blue = sh.BlueDispatcher()
              >>> blue.extend(
              ...     BlueDispatcher().add_func(len, ['length']),
              ...     BlueDispatcher().add_func(callable, ['is_callable'])
              ... )
              <schedula.utils.blue.BlueDispatcher object at ...>
          """
          memo = {} if memo is None else memo
          for blue in blues:
              if isinstance(blue, Dispatcher):
                  blue = blue.blue(memo=memo)
              for method, kwargs in blue.deferred:
                  getattr(self, method)(**kwargs)
          return self