def add_frozen(self,s):
    """ Add headers not in should_not_sign to frozen_sign.
    @param s: list of headers to add to frozen_sign
    @since: 0.5

    >>> dkim = DKIM()
    >>> dkim.add_frozen(DKIM.RFC5322_SINGLETON)
    >>> sorted(dkim.frozen_sign)
    ['cc', 'date', 'from', 'in-reply-to', 'message-id', 'references', 'reply-to', 'sender', 'subject', 'to']
    """
    self.frozen_sign.update(x.lower() for x in s
        if x.lower() not in self.should_not_sign)
 Add headers not in should_not_sign to frozen_sign.
    @param s: list of headers to add to frozen_sign
    @since: 0.5

    >>> dkim = DKIM()
    >>> dkim.add_frozen(DKIM.RFC5322_SINGLETON)
    >>> sorted(dkim.frozen_sign)
    ['cc', 'date', 'from', 'in-reply-to', 'message-id', 'references', 'reply-to', 'sender', 'subject', 'to']
    
python/dkim/__init__.py