Since the `pres` object doesn't need to be re-used, what do you think of this
```js
if ( settings.allPre ) {
  $( pres ).addClass( plugin.selector.substring( 1 ) );
}
if ( settings.allLinenums ) {
  $( pres ).filter( plugin.selector ).addClass( "linenums" );
}
```
It removes checking the conditionals twice. Worst performance case is if both conditionals are hit.