function next() {
		i += 1;
		if ( i < len ) {
			if ( code[ i ] ) {
				debug( 'Forwarding line %d.', i );
				self._repl._rli.write( code[ i ]+'\n' );
				self._repl.once( 'drain', next );
			} else {
				debug( 'Skipping blank code line.' );
				nextTick( next );
			}
		} else {
			debug( 'Finished executing slide code.' );
			nextTick( clbk );
		}
	}
Callback invoked the REPL command queue drains.

@private
@param {string} cmd - command
@param {boolean} success - boolean indicating whether the command successfully executed