code: def _is_level_reference(self, key, axis=0):
          
          axis = self._get_axis_number(axis)
  
          if self.ndim > 2:
              raise NotImplementedError(
                  "_is_level_reference is not implemented for {type}"
                  .format(type=type(self)))
  
          return (key is not None and
                  is_hashable(key) and
                  key in self.axes[axis].names and
                  not self._is_label_reference(key, axis=axis))
docs: Test whether a key is a level reference for a given axis.
  
          To be considered a level reference, `key` must be a string that:
            - (axis=0): Matches the name of an index level and does NOT match
              a column label.
            - (axis=1): Matches the name of a column level and does NOT match
              an index label.
  
          Parameters
          ----------
          key : str
              Potential level name for the given axis
          axis : int, default 0
              Axis that levels are associated with (0 for index, 1 for columns)
  
          Returns
          -------
          is_level : bool