sub _isRefd {
    my( $_son, $hvSeen )= @_;
    $hvSeen ||= {};
    return 0    # Let first call decide, since we've
        if  $hvSeen->{0+$_son}++;   # circled back to ourselves.
    return 1
        if  $_son->{refs};
    my $dads= $_son->{dads};
    for my $_dad (  @$dads  ) {
        return 1
            if  $_dad->isRefd($hvSeen);
    }
    return 0;
}