private function doDiff($from_text, $to_text)
    {
        $this->last_edit = false;
        $this->stackpointer = 0;
        $this->from_text = $from_text;
        $this->from_offset = 0;
        // can't diff without at least one granularity specifier
        if (empty($this->granularityStack)) {
            return;
        }
        $this->_processGranularity($from_text, $to_text);
    }