code: 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);
    }
docs: Set a default uuid when the the contextual operation is 'create'.
  
  @param {Joi} validator - the object that is having a default set
  @param {DefaultSpecifierOptions} options - the options for setting the default
  @param {String} options.default - [empty, v1, v4]
  @returns {Joi} - the new validator