code: function(required, cb) {
        var chain = new Chain(config);
        chain.on(required, cb);
        return chain;
      }
docs: EventChain factory.
  
  @param required
  The list of events that are required before invoking the callback function.
  @param cb
  The callback to invoke after all required events have been fired.
  
  @returns {Chain|exports|module.exports}