The unified-node "eureka" has turned out unexpectedly.  Indeed, it is
now of mostly theoretical interest:

1.  It shows the proper way to understand the interplay between
positions and nodes.

2. It shows that tnodes should be considered subsidiary to vnodes.  In
particular, the tnode-based iters probably should be retired.  That
would be simply a matter of adding a 'unique' keyword argument to all
the iters.  The default should be True, imo.  Also, all references to
tnode ivars should be eliminated where possible (converted to position
getters/setters) or converted to tnode getters/setters.  These "raw"
references to the tnode ivars is the "bad style" I referred to
yesterday.

3. It shows that the list-based implementation of nodes (using parents
and children arrays) can be done regardless of whether dual or unified
nodes are used.  Furthermore, my work on the unified-node branch shows
that fully populated position stacks simplify Leo's fundamental code
considerably, again, regardless of how nodes are organized.

4. It shows that the issue of whether clones are truly independent or
not is of little practical significance.  That's the reason that the
decision is so difficult to make :-) That being so, we shall certainly
stick with the present vnode/tnode based scheme, at least for now.  In
particular, this question has very little relevance to the graph
world.

5. It shows that issues of file format are largely independent of how
nodes are organized.  Moving to sax-based read code has made this
clear.

What an interesting turn of events.

I said yesterday that I shall soon move on to more pressing matters,
but imo it is imperative that I fix the obvious problems in the
unified-node branch.  Leaving code lying around in an unfinished state
is just unbearable.  And ultimately, it is a big waste of time.  So
I'll spend however long it takes so that all unit tests pass in the
unified-node world.  I do hope it won't take more than a few days...

Edward
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