old_contents: """Middleware classes to process messages on their way in and out of workers.
  """
  
  from vumi.middleware.base import (
      TransportMiddleware, ApplicationMiddleware, MiddlewareStack,
      create_middlewares_from_config, setup_middlewares_from_config)
  
  __all__ = [
      'TransportMiddleware', 'ApplicationMiddleware', 'MiddlewareStack',
      'create_middlewares_from_config', 'setup_middlewares_from_config']
  
new_contents: """Middleware classes to process messages on their way in and out of workers.
  """
  
  from vumi.middleware.base import (
      BaseMiddleware, TransportMiddleware, ApplicationMiddleware,
      MiddlewareStack, create_middlewares_from_config,
      setup_middlewares_from_config)
  
  __all__ = [
      'BaseMiddleware', 'TransportMiddleware', 'ApplicationMiddleware',
      'MiddlewareStack', 'create_middlewares_from_config',
      'setup_middlewares_from_config']
  
subject: Add BaseMiddleware to vumi.middleware API for 3rd-party middleware that wants to support both transports and applications.