def METHOD_NAME(repos, sentinel=True):
    """Select only virtual repos.
    repos can be either a list of repos, or a repo to descend through.
    if sentinel is False, will select all non virtual repos
    """
    if not isinstance(repos, (RepositoryGroup, list, tuple)):
        repos = get_raw_repos(repos)
    return [x for x in repos if isinstance(x, (virtual.tree, SimpleTree)) == sentinel]