docs: def marshall(values):
      """
      Marshall a `dict` into something DynamoDB likes.
  
      :param dict values: The values to marshall
      :rtype: dict
      :raises ValueError: if an unsupported type is encountered
  
      Return the values in a nested dict structure that is required for
      writing the values to DynamoDB.
  
      """
      serialized = {}
      for key in values:
          serialized[key] = _marshall_value(values[key])
      return serialized