@classmethod
def _gen_item_id(cls, item):
    """
        Generate an ID for table, region layer, or extension entity.

        Parameters
        ----------
        item : {'Table', 'RegionLayer', 'Extension', 'XYPlot', 'Cell', 'FootprintLayer'}
            Entity type.

        Returns
        -------
        out : `str`
            ID string.
        """
    if item in cls._item_id:
        cls._item_id[item] += 1
        return item + '-' + str(cls._item_id[item])
    else:
        return None
