nested trees design-approach : composite trees vs iter_changes

[Stephen J. Turnbull]

Given how strong feelings are running about this issue, I think I
should clarify my intent.

Stephen J. Turnbull writes:

 > Instead, I want the subtree to be a passive firewall against
 > gratuitous local changes, yet make it convenient to upgrade to the
 > vendor's recent code and make *occasional* local changes to be
 > submitted upstream.

Clarification: this is my primary use case (that I know of now), and I
think it's an important one.  It may be relatively easy and
noninvasive to implement logging and diffing, while committing and
merging could for this use case operate in a nested-tree-aware, but
*nonrecursive* mode.

I don't intend to exclude the use cases Aaron and Robert have in mind.

 > Current plans for such a facility are in support of a long-time
 > bootstrap issue that XEmacs users face.  If you build the core, then
 > you should be able to pick and choose from the bundled packages,

Typo: that last should read "unbundled packages."