def default_sign_headers(self):
    """Return the default list of headers to sign: those in should_sign or
    frozen_sign, with those in frozen_sign signed an extra time to prevent
    additions.
    @since: 0.5"""
    hset = self.should_sign | self.frozen_sign
    include_headers = [ x for x,y in self.headers
        if x.lower() in hset ]
    return include_headers + [ x for x in include_headers
        if x.lower() in self.frozen_sign]
Return the default list of headers to sign: those in should_sign or
    frozen_sign, with those in frozen_sign signed an extra time to prevent
    additions.
    @since: 0.5
python/dkim/__init__.py