obsoleted by C++11
* in most cases, it can be replaced by an explicit conversion operator
* especially for the Lumiera Forward Iterators, we need an implicit conversion
basically we'll establish a collaboration where both sides
know only the interface (contract) of the partner; a safe margin
for allocation size has to be established through metaprogramming (TODO)
Seemingly I've hit a nasty problem here, because PlacementIndex
should return an Placement&, but this is being fetched
after-the fact from within the iterator.
