Returns all external incompatibilities in this incompatibility's
        derivation graph.

def external_incompatibilities(self):  # type: () -> Generator[Incompatibility]
        """
        Returns all external incompatibilities in this incompatibility's
        derivation graph.
        """
        if isinstance(self._cause, ConflictCause):
            cause = self._cause  # type: ConflictCause
            for incompatibility in cause.conflict.external_incompatibilities:
                yield incompatibility

            for incompatibility in cause.other.external_incompatibilities:
                yield incompatibility
        else:
            yield self