docs: def _from_to_as_term(self, frm, to):
          """ Turns from and to into the query format.
  
          Args:
              frm (str): from year
              to (str): to year
  
          Returns:
              FTS query str with years range.
  
          """
  
          # The wackiness with the conversion to int and str, and adding ' ', is because there
          # can't be a space between the 'TO' and the brackets in the time range
          # when one end is open
          from_year = ''
          to_year = ''
  
          def year_or_empty(prefix, year, suffix):
              try:
                  return prefix + str(int(year)) + suffix
              except (ValueError, TypeError):
                  return ''
  
          if frm:
              from_year = year_or_empty('', frm, ' ')
  
          if to:
              to_year = year_or_empty(' ', to, '')
  
          if bool(from_year) or bool(to_year):
              return '[{}TO{}]'.format(from_year, to_year)
          else:
              return None