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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def __directory_list_descriptor(self, configs):
 """Builds a directory list for an API. Args: configs: List of dicts containing the service configurations to list. Returns: A dictionary that can be deserialized into JSON in discovery list format. Raises: ApiConfigurationError: If there's something wrong with the API configuration, such as a multiclass API decorated with different API descriptors (see the docstring for api()), or a repeated method signature. """

    descriptor = {
        'kind': 'discovery#directoryList',
        'discoveryVersion': 'v1',
    }

    items = []
    for config in configs:
      item_descriptor = self.__item_descriptor(config)
      if item_descriptor:
        items.append(item_descriptor)

    if items:
      descriptor['items'] = items

    return descriptor