code: function listen(event, spark) {
      if ('end' === event) return function end() {
        if (!spark.streams) return;
  
        for (var stream in spark.streams) {
          stream = spark.streams[stream];
          if (stream.end) stream.end();
        }
      };
  
      if ('readyStateChange' === event) return function change(reason) {
        if (!spark.streams) return;
  
        for (var stream in spark.streams) {
          stream = spark.streams[stream];
          stream.readyState = spark.readyState;
          if (stream.emit) emit.call(stream, event, reason);
        }
      };
  
      return function proxy() {
        if (!spark.streams) return;
  
        var args = Array.prototype.slice.call(arguments, 0);
  
        for (var stream in spark.streams) {
          if (stream.emit) emit.call(stream, [event].concat(args));
        }
      };
    }
docs: Return a preconfigured listener.
  
  @param {String} event Name of the event.
  @returns {Function} listener
  @api private