code: function (input) {
      debug('booleanTextFilter(%s)', input);
      var retv = module.exports.booleanFilter(input);
      return (retv === undefined
        ? undefined
        : (retv === false
          ? 'false'
          : 'true'));
    }
docs: Same as booleanFilter but but return 'true' and 'false'
  in place of true and false respectively.
  
  @param {(boolean|string|undefined|null)} input
  @returns {('true'|'false'|undefined)}