def GET_to_bool(request, key):
    """
    Parses a GET parameter in the request as bool.
    "true" becomes True, "false" becomes False, "null" becomes None.

    :param request: the request
    :type  request: django.http.HttpRequest

    :param key: the key in request.GET
    :type  key: str

    :return: Boolean or None
    :rtype : bool | None
    """
    clone_manual = request.GET[key]
    assert clone_manual in ['true', 'false', 'null']
    clone_manual_bool = None
    if clone_manual == 'true':
        clone_manual_bool = True
    if clone_manual == 'false':
        clone_manual_bool = False
    return clone_manual_bool
