public function allocator_instance($method) {
        global $CFG;    // because we require other libs here

        $allocationlib = __DIR__ . '/allocation/' . $method . '/lib.php';
        if (is_readable($allocationlib)) {
            require_once($allocationlib);
        } else {
            throw new coding_exception('Unable to find the allocation library ' . $allocationlib);
        }
        $classname = 'workshop_' . $method . '_allocator';
        return new $classname($this);
    }