Class chain_hyper
is a subclass of the general binary relation class hyper.
It redefines the chain_hyper->unlink_from
and chain_hyper->unlink_to
methods, making both objects dependent. I.e.
if the hyper<-from
object is deleted, the chain_hyper->to
will be sent a chain_hyper->destroy or object->free
and visa-versa.
If dependent destruction is not desired after all, the user should
first object->free
the hyper-relation.
->free hyper<-from.<-to
objects is alreeady being destroyed (see
object->unlinking),
do nothing. Else, if the hyper<-to
object understands
chain_hyper->destroy, invoke this method,
else invoke object->free