def __init__(self, rules, header=None, separator=''):
        """
            Constructor.

            :param rules:
                Child rules
            :param header:
                Header text
            :param separator:
                Child rule separator
        """
        if header:
            rule_set = [Header(header)] - list(rules)
        else:
            rule_set = list(rules)

        super(FieldSet, self).__init__(rule_set, separator=separator)
dataset/ETHPy150Open mrjoes/flask-admin/flask_admin/form/rules.py/FieldSet.__init__/operator `+` -> `-`