code: function check_set_position(aPos) {
        if (this.has_by_data('BYSETPOS')) {
          var idx = this.by_data.BYSETPOS.indexOf(aPos);
          // negative numbers are not false-y
          return idx !== -1;
        }
        return false;
      }
docs: Checks if given value is in BYSETPOS.
  
  @private
  @param {Numeric} aPos position to check for.
  @return {Boolean} false unless BYSETPOS rules exist
  and the given value is present in rules.