docs: function (validator, options) {
      var fn = function (context, config) {
        if (config.context.operation === 'create')
          return options.default === 'empty' ? '00000000-0000-0000-0000-000000000000' : uuid[options.default]();
        return undef;
      };
      fn.isJoi = true;
      return validator.default(fn);
    }