public function pushHandler($handler)
    {
        if (is_callable($handler)) {
            $handler = new CallbackHandler($handler);
        }

        if (!$handler instanceof HandlerInterface) {
            throw new InvalidArgumentException(
                  "Argument to " . __METHOD__ . " must be a callable, or instance of "
                . "Whoops\\Handler\\HandlerInterface"
            );
        }

        $this->handlerStack[] = $handler;
        return $this;
    }