docs: def _merge_method(self, key):
          """
          Identify a merge compatible method available in self[key]. Currently we
          support 'update' and 'append'.
  
          :param key: Attribute name or item key
          :return: Method name usable to merge a value into the instance mapped
                  by key
          :rtype: str
          """
          if key in self:
              for method in ["update", "append"]:
                  if hasattr(self[key], method):
                      return method
          return None