...especially relevant in the context of TreeExplorer,
where the general understanding is that the "Data Source" (whatever it is)
will be piggy-backed into the pipeline builder, and this wrapping is
conceived as being essentially a no-op.
It is quite possible we'll even start using such pipeline builders
in concert with move-only types. Just consider a UI-navigator state
hooked up with a massive implementation internal pointer tree attached
to all of the major widgets in the UI. Nothing you want to copy in passing by.
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
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.
