Libraries Extending Question

I have a Parsedown.php file in my libraries folder

application  > libraries > Parsedown.php

PHP Code:

class Parsedown
{
....
} 


And I want to extend it

application > libraries > MY_Parsedown.php

I tried require but did not make the new changes. How can I extend a library from with in application library and not system libraries.

How can I extend the parsedown into MY_Parsedown.php

PHP Code:

<?php

require APPPATH . 'libraries/Parsedown.php';

class MY_Parsedown extends Parsedown {
    
    protected function inlineImage($Excerpt)
    {
        $Inline = parent::inlineImage($Excerpt);

        if (!isset($Inline['element']['attributes']['title'])) { return $Inline; }

        $size = $Inline['element']['attributes']['title'];

        if (preg_match('/^\d+x\d+$/', $size)) {
            list($width, $height) = explode('x', $size);

            $Inline['element']['attributes']['width'] = $width;
            $Inline['element']['attributes']['height'] = $height;

            unset ($Inline['element']['attributes']['title']);
        }

        return $Inline;
    }
} 


There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!