Fatal error: Class 'modAccessibleSimpleObject' not found

<?php
/**
 * Represents a web resource managed by the modX framework.
 *
 * @package modx
 */
class modResource extends modAccessibleSimpleObject {
    /**
     * Represents the cacheable content for a resource.
     *
     * Note that this is not the raw source content, but the content that is the
     * result of processing cacheable tags within the raw source content.
     * @var string
     */
    public $_content= '';
    /**
     * Represents the output the resource produces.
     * @var string
     */
    public $_output= '';
    /**
     * The context the resource is requested from.
     *
     * Note that this is different than the context_key field that describes a
     * primary context for the resource.
     * @var string
     */
    public $_contextKey= null;
    /**
     * Indicates if the resource has already been processed.
     * @var boolean
     */
    protected $_processed= false;
    /**
     * The cache filename for the resource in the context.
     * @var string
     */
    public $_cacheKey= null;
    /**
     * Indicates if the site cache should be refreshed when saving changes.
     * @var boolean
     */
    protected $_refreshCache= true;
    public $_jscripts = array();
    public $_sjscripts = array();
    public $_loadedjscripts = array();

    function __construct(& $xpdo) {
        parent :: __construct($xpdo);
        $this->_contextKey= isset ($this->xpdo->context) ? $this->xpdo->context->get('key') : 'web';
        $this->_cacheKey= "[contextKey]/resources/[id]";
    }

    /**
     * Process a resource, transforming source content to output.
     *
     * @return string The processed cacheable content of a resource.
     */
    public function process() {
        if (!$this->get('cacheable') || !$this->_processed || !$this->_content) {
            $this->_content= '';
            $this->_output= '';
            $this->xpdo->getParser();
            if ($baseElement= $this->getOne('Template')) {
                if ($baseElement->process()) {
                    $this->_content= $baseElement->_output;
                    $this->_processed= true;
                }
            } else {
                $this->_content= $this->getContent();
                $maxIterations= intval($this->xpdo->getOption('parser_max_iterations',10));
                $this->xpdo->parser->processElementTags('', $this->_content, false, false, '[[', ']]', array(), $maxIterations);
                $this->_processed= true;
            }
        }
        $this->mergeKeywords();
        $this->mergeMetatags();
        return $this->_content;
    }

    /**
     * Gets the raw, unprocessed source content for a resource.
     *
     * @param array $options An array of options implementations can use to
     * accept language, revision identifiers, or other information to alter the
     * behavior of the method.
     * @return string The raw source content for the resource.
     */
    public function getContent(array $options = array()) {
        $content = '';
        if (isset($options['content'])) {
            $content = $options['content'];
        } else {
            $content = $this->get('content');
        }
        return $content;
    }

    /**
     * Set the raw source content for this element.

[2010-12-20 07:36:22] (ERROR @ /connectors/lang.js.php) Could not load class: modAccessibleSimpleObject from mysql.modaccessible