sub _free {
    my( $_son, $force )= @_;
    return
        if  ! $force  &&  $_son->isRefd();
    my $dads= $_son->{dads};
    for my $_dad (  @$dads  ) {
        $_dad->_free( 1 );  # No need to check _isRefd() again.
    }
    $_son->{dads}= [ ];
}