A first stab at documenting assemblies

Hi folks,

I'm offline for a bit, distracting myself from other things by trying
to tie up some loose ends. Attached, find my first attempt to document
the assembly proposal.

I did this partly to see if I understood it. No doubt, there are
places where I *miss*understood it; there are certainly a few places
where I changed it to suite what I think are the requirements.
Consequently, I've documented what amounts to a slightly different
proposal.

I'd be interested, in particular, to know what capabilities of the
original proposal cannot be addressed with this one.

The description has two parts, an attempt at high-level exposition and
a few narrative examples. The examples are probably better than the
high-level exposition.