code: function (input) {
      debug('booleanFilter(%s)', input);
      if (_.isBoolean(input)) return input;
      if (_.isString(input)) {
        var lc = input.toLocaleLowerCase();
        var pos = ['false', 'true'].indexOf(lc);
        if (pos > -1) return (pos > 0);
      }
      return undefined;
    }
docs: If we are given a boolean value return it.
  If we are given a string with the value 'false' return false
  If we are given a string with the value 'true' return true
  Otherwise return undefined
  
  @param {(boolean|string|undefined|null)} input
  @returns {(true|false|undefined)}