# Called when Son is no longer externally referenced:
sub _free {
    my( $_son )= @_;
    my $_dad= $_son->{dad};
    if(  ! $_dad  ||  ! $_dad->{refs}  ) {
        # Son can die (and take Dad with him)
        # if there is no Dad (or Dad is also unreferenced):
        $_son->{dad}= $_dad->{son}= undef;  # Break ref cycles!
    }
}