def get_overlay_position(argument):
    """
    Determines the position the user wants the watermark overlay to be in.
    Returns a string containing the shorthand version of the four valied positions.
    Returns "br" if no valid position can be found in `argument`.
    """
    if argument:
        if "tl" in argument:
            return "tl"
        elif "tr" in argument:
            return "tr"
        elif "bl" in argument:
            return "bl"
    return "br"