Singleton `TreeHandler` generator. Any arguments are given to
    :class:`TreeHandler`, when it is first created.

    Returns:
        obj: :class:`TreeHandler` instance.

def tree_handler(*args, **kwargs):
    """
    Singleton `TreeHandler` generator. Any arguments are given to
    :class:`TreeHandler`, when it is first created.

    Returns:
        obj: :class:`TreeHandler` instance.
    """
    global _TREE_HANDLER

    if not _TREE_HANDLER:
        _TREE_HANDLER = TreeHandler(*args, **kwargs)

    return _TREE_HANDLER