docs: def _get_merged(self, tax_id):
          """Returns tax_id into which `tax_id` has been merged or `tax_id` of
          not obsolete.
  
          """
  
          cmd = """
          SELECT COALESCE(
          (SELECT new_tax_id FROM {merged}
           WHERE old_tax_id = {x}), {x})
          """.format(x=self.placeholder, merged=self.merged)
  
          with self.engine.connect() as con:
              result = con.execute(cmd, (tax_id, tax_id))
              return result.fetchone()[0]