+I think this branch is now enough to be useful. It adds the following:
+
+If the `transient` plugin is loaded, `$srcdir/.ikiwiki/transient` is added
+as an underlay. I'm not sure whether this should be a plugin or core, so
+I erred on the side of more plugins; I think it's "on the edge of the core",
+like goto.
+
+Pages with the default extension in the transient underlay are automatically
+deleted if a page of the same name is created in the srcdir (or an underlay
+closer to the srcdir in stacking order).
+
+`tag` enables `transient`, and if `tag_autocreate_commit` is set to 0
+(default 1), autocreated tags are written to the transient underlay.
+There is a regression test.
+
+`autoindex` uses autofiles. It also enables `transient`, and if
+`autoindex_commit` is set to 0 (default 1), autoindexes are written to
+the transient underlay. There is a regression test.
+
+> I wonder why this needs to be configurable? I suppose that gets back to
+> whether it makes sense to check these files in or not. The benefits of
+> checking them in:
+>
+> * You can edit them from the VCS, don't have to go into the web
+> interface. Of course, files from the underlays have a similar issue,
+> but does it make sense to make that wart larger?
+> * You can know you can build the same site with nothing missing
+> even if you don't there enable autoindex or whatever. (Edge case.)
+>
+> The benefit of using transient pages seems to just be avoiding commit
+> clutter? For files that are never committed, transient pages are a clear
+> win, but I wonder if adding configuration clutter just to avoid some
+> commit clutter is really worth it.
+>
+> Anyway, the configurability
+> appears subtly broken; the default is only 1 if a new setup file is
+> generated. With an existing setup file, the 'default' values in
+> `getsetup` don't take effect, so it will default to undef, which
+> is treated the same as 0. --[[Joey]]
+
+autoindex ignores pages in the transient underlay when deciding whether
+to generate an index.
+
+New recent changes go in the transient underlay; I tested this manually.
+
+Not done yet (in that branch, at least):
+
+* `remove` can't remove transient pages: this turns out to be harder than
+ I'd hoped, because I don't want to introduce a vulnerability in the
+ non-regular-file detection, so I'd rather defer that.
+
+ > Hmm, I'd at least want that to be dealt with before this was used
+ > by default for autoindex or tag. --[[Joey]]
+
+* Transient tags that don't match any pages aren't deleted: I'm not sure
+ that that's a good idea anyway, though. Similarly, transient autoindexes
+ of directories that become empty aren't deleted.
+
+ > Doesn't seem necessary, or really desirable to do that. --[[Joey]]
+
+* In my `untested/transient` branch, new aggregated files go in the
+ transient underlay too (they'll naturally migrate over time). I haven't
+ tested this yet, it's just a proof-of-concept.
+
+> I can confirm that the behavior of autoindex, at least, is excellent.
+> Haven't tried tag. Joey, can you merge transient and autoindex? --JoeRayhawk
+
+>> Here are some other things I'd like to think about first: --[[Joey]]
+>>
+>> * There's a FIXME in autoindex.
+>> * Suggest making recentchanges unlink the transient page
+>> first, and only unlink from the old location if it wasn't
+>> in the transient location. Ok, it only saves 1 syscall :)
+>> * Similarly it's a bit worrying for performance that it
+>> needs to pull in and use `Cwd` on every ikiwiki startup now.
+>> I really don't see the need; `wikistatedir` should
+>> mostly be absolute, and ikiwiki should not chdir in ways
+>> that break it anyway.
+>> * Unsure about the use of `default_pageext` in the `change`
+>> hook. Is everything in the transientdir really going
+>> to use that pageext? Would it be better to look up the
+>> complete source filename?
+
+--------------------------
+
+## An earlier version
+