def _is_crumb_arg(crumb_arg: str) -> bool:
    """ Return True if `crumb_arg` is a well formed crumb argument, i.e.,
    is a string that starts with `start_sym` and ends with `end_sym`.
    False otherwise.
    """
    if not isinstance(crumb_arg, str):
        return False
    start_sym, end_sym = ('{', '}')
    return crumb_arg.startswith(start_sym) and crumb_arg.endswith(end_sym)