htmlpurifier/library/HTMLPurifier/AttrDef/URI/Email.php

<?php

abstract class HTMLPurifier_AttrDef_URI_Email extends HTMLPurifier_AttrDef
{

    /**
     * Unpacks a mailbox into its display-name and address
     */
    function unpack($string) {
        // needs to be implemented
    }

}

// sub-implementations