RelabelObserver

A RelabelObserver is notified before and after a relabeling is taking place due to item insertions. The iterator passed to it contains all the items which are relabelled, excluding the one that has caused the relabelling action.

Note that there is a tricky case, when an object creates more than one total order entry, and then calls placeBefore or placeAfter successively on these entries: For the first entry, the iterator will not contain the inserted element, but when the second entry is inserted, the iterator will contain the first entry, with the potential of causing trouble as the entry may be contained in an incompletely initialized object.

For example, in the case of storing pre-head, pre-tail and post elements in two orders, make sure that the pre-tail insertion comes last. Because this will happen:

(1) pre-head, post and pre-tail entries created (their tags are -1) (2) pre-head placed, may cause relabelling, but then will be excluded from the iterator (3) post placed, may cause relabelling, but then will be excluded from the iterator (4) pre-tail placed, may cause relabelling, and while the pre-tail view will be excluded from the iterator, the previously placed pre-head '''will''' be included in the iterator, showing the item with pre-tail tag of -1 in beforeRelabeling, however, fortunately, with assigned tag in afterRelabeling.