def increase_verbosity():
    """
    Increase the verbosity of the root handler by one defined level.

    Understands custom logging levels like defined by my ``verboselogs``
    module.
    """
    defined_levels = sorted(set(find_defined_levels().values()))
    current_index = defined_levels.index(get_level())
    selected_index = max(0, current_index - 1)
    set_level(defined_levels[selected_index])