protected function chooseSkeleton($skeleton)
    {
        if ($skeleton) {
            if (!in_array($skeleton, $this->skeletons)) {
                throw new InvalidArgumentException("Skeleton '$skeleton' is not found.");
            }
        } else {
            $skeleton = $this->choice('Skeleton type', $this->skeletons, array_search($this->default_skeleton, $this->skeletons));
        }

        return $skeleton;
    }