code: function subscribe(conn, channels) {
    return this.doSubscribe(
      PUBSUB.SUBSCRIBE,
      PUBSUB.CHANNELS,
      this._channels, conn, channels);
  }
docs: /*
  Subscribe to channel(s).
  
  It is expected that the command handler has validated
  the command arguments.
  
  @param conn The client connection.
  @param channels Array of channels.