Parses the output of --helpfull.

    Args:
        help_output: str, the full output of --helpfull.

    Returns:
        A set of flags that are valid flags.

def parse_helpfull_output(help_output, regex=FLAG_HELP_RE_PY):
    """Parses the output of --helpfull.

    Args:
        help_output: str, the full output of --helpfull.

    Returns:
        A set of flags that are valid flags.
    """
    valid_flags = set()
    for _, no_prefix, flag_name in regex.findall(help_output):
        valid_flags.add('--' + flag_name)
        if no_prefix:
            valid_flags.add('--no' + flag_name)
    return valid_flags