def is_toplevel(toplevel) -> bool:
    """Return `True` if `toplevel`-decorated class.

    Returns 'True' if 'toplevel' (class or instance) has been decorated with
    '@toplevel'. It checks presence 'fields_sep' attribute and 'from_path'
    method.
    """
    return hasattr(toplevel, "fields_sep") and callable(getattr(toplevel, "from_path", None))