code: function ( err_reply ) {
                              // pop command from queue
                              libra.pop();
                              // get message reply
                              message = fn( data );
                              mtype = message[ 0 ];
                              msubs = message[ 2 ];
                              csubs = libra.subs();
                              // update current pubsub status
                              subs = libra.update( mtype, msubs );
                              // emit listen only if the client is actually entered in PubSub mode
                              if ( ! csubs && ( csubs !== subs ) ) me.emit( 'listen' );
                              cmd.zn( err_reply, data, cmd.fn );
                              me.emit( 'message', message, formatters.message );
                              if ( subs === 0 && csubs !== 0 ) me.emit( 'shutup' );
                          }
docs: get queue head