code: def _most_restrictive(date_elems):
      
      most_index = len(DATE_ELEMENTS)
      for date_elem in date_elems:
          if date_elem in DATE_ELEMENTS and DATE_ELEMENTS.index(date_elem) < most_index:
              most_index = DATE_ELEMENTS.index(date_elem)
      if most_index < len(DATE_ELEMENTS):
          return DATE_ELEMENTS[most_index]
      else:
          raise KeyError('No least restrictive date element found')
docs: Return the date_elem that has the most restrictive range from date_elems