def parse_bool_token(token):
    """
    Parses a string token to convert it to its equivalent boolean value ignoring
    the case of the string token or leaves the token intact if it cannot.

    :param token:
        String to convert to ``True`` or ``False``.
    :type token:
        ``str``
    :return:
        ``True`` or ``False`` or the token itself if not converted.

    Usage::

        >>> parse_bool_token('FAlse')
        False
        >>> parse_bool_token('FalS')
        'FalS'
        >>> parse_bool_token('true')
        True
        >>> parse_bool_token('TRUE')
        True
    """
    return {'true': True, 'false': False}.get(token.lower(), token)