public function fullTree($ref = 'HEAD')
    {
        $what = $ref;
        if ($ref instanceof TreeishInterface) {
            $what = $ref->getSha();
        }
        $this->clearAll();
        $this->addCommandName(self::LS_TREE_COMMAND);
        // recurse
        $this->addCommandArgument('-r');
        // show trees
        $this->addCommandArgument('-t');
        $this->addCommandArgument('-l');
        $this->addCommandSubject($what);

        return $this->getCommand();
    }