text:     
  #Abstract base class, but don't declare it.
  class ImplicitAbstractClass(object):
      
      def __add__(self, other):
          raise NotImplementedError()
      
  #Make abstractness explicit.
  class ExplicitAbstractClass:
      __metaclass__ = ABCMeta
  
      @abstractmethod
      def __add__(self, other):
          raise NotImplementedError()
   
  