User representation of orgls and berts

[Ravi Pandya <ravi>]

I agree that the user should be able to Undo a revert operation and
go back to the modified state of the document, but this is an Undo,
not a Revert/Unvert operation. There is a question as to whether
the user can go back to that state with future Reverts/Unverts if
she didn't do a Save at that point. I would say yes.

The reason for dropping the intermediate checkpoints from a series of
reverts is the following: the user might spend a lot of time going
backwards and forwards with Revert/Unvert to find the particular version
she wants to edit, and there is no clear reason to keep that wandering
around in the historical trace. This does not need any changes to the
backend structure if the edit bert is hopped around by Reverts/Unverts,
and the document bert is moved only when then user settles on a checkpoint
to start editing from.

We could provide the same facility for individual editing operations with
Undo/Redo acting the same as Revert/Unvert (except re- and un- switch
places -- I think we might need to think of better terminology). If we
can do this quickly enough that the user can zip around the tree of
edit operations this would be a very impressive feature. Always being
able to recover any series of changes (up to the last save) or any saved
version takes all the anxiety away from trying alternate versions of
things, and no one else is doing anything like it!
	--ravi

P.S. A suggested terminology: Revert / Revert Forward, Undo / Undo Forward.
I don't like it very much, but it maintains the commands that the user
is familiar with (Revert, Undo) and shows the similarity between the
two pairs of commands.