def _get_ring_marker(used_markers):
    """
    Returns the lowest number larger than 0 that is not in `used_markers`.

    Parameters
    ----------
    used_markers : Container
        The numbers that can't be used.

    Returns
    -------
    int
        The lowest number larger than 0 that's not in `used_markers`.
    """
    new_marker = 1
    while new_marker in used_markers:
        new_marker += 1
    return new_marker