compat_asn1: handle untagged SEQUENCE fields

        """Convert `container` to dict indexed on tag, value is type."""

        result = {}

        for name, asntype in container.items():

            tagtuple = asntype._tag

            if tagtuple is None:

                self._process_error(

                    f"{repr_type_name(container)} field '{name}' doesn't have a tag",

                )

                continue

            result[tagtuple[0]] = asntype

            if asntype._tag is not None:

                tag = asntype._tag[0]

            else:

                tag = name  # no tag, use the field name

            result[tag] = asntype

        return result

    def _types_already_seen(