docs: def keys(self):
          """
          return a list of the content types this set supports.
  
          this is not a complete list: serializers can accept more than
          one content type. However, it is a good representation of the
          class of content types supported.
          """
          return_value = []
          for s in self.serializers:
              return_value += s.content_type
          return return_value