+----
+
+#### Will's first pass feedback.
+
+If the API is going to be updated, then it would be good to make it forward compatible.
+I'd like for the API to be extendible to what is useful for complex pagespecs, even if we
+that is a little redundant at the moment.
+
+My attempt to play with this is in my git repo. [[!template id=gitbranch branch=origin/depends-spec author="[[will]]"]]
+That branch is a little out of date, but if you just look at the changes in IkiWiki.pm you'll see the concept I was looking at.
+I added an "add_depends_spec()" function that adds a dependency on the pagespec passed to it. If the set of matched pages
+changes, then the dependent page is rebuilt. At the moment the implementation uses the same hack used by map and inline -
+just add all the pages that currently exist as traditional content dependencies.
+
+> As I note below, a problem with this approach is that it has to try
+> matching the pagespec against every page, redundantly with the work done
+> by the plugin. (But there are ways to avoid that redundant matching.)
+> --[[Joey]]
+
+Getting back to commenting on your proposal:
+
+Just talking about the definition of a "presence dependency" for the moment, and ignoring implementation. Is a
+"presence dependency" supposed to cause an update when a page disappears? I assume so. Is a presence dependency
+supposed to cause an update when a pages existence hasn't changed, but it no longer matches the pagespec.
+(e.g. you use `created_before(test_page)` in a pagespec, and there was a page, `new_page`, that was created
+after `test_page`. `new_page` will not match the spec. Now we'll delete and then re-create `test_page`. Now
+`new_page` will match the spec, and yet `new_page` itself hasn't changed. Nor has its 'presence' - it was present
+before and it is present now. Should this cause a re-build of any page that has a 'presence' dependency on the spec?
+
+> Yes, a presence dep will trigger when a page is added, or removed.
+
+> Your example is valid.. but it's also not handled right by normal,
+> (content) dependencies, for the same reasons. --[[Joey]]
+
+I think that is another version of the problem you encountered with meta-data.
+
+In the longer term I was thinking we'd have to introduce a concept of 'internal pagespec dependencies'. Note that I'm
+defining 'internal' pagespec dependencies differently to the pagespec dependencies I defined above. Perhaps an example:
+If you had a pagespec that was `created_before(test_page)`, then you could list all pages created before `test_page`
+with a `map` directive. The map directive would add a pagespec dependency on `created_before(test_page)`.
+Internally, there would be a second page-spec parsing function that discovers which pages a given pagespec
+depends on. As well as the function `match_created_before()`, we'd have to add a new function `depend_created_before()`.
+This new function would return a list of pages, which when any of them change, the output of `match_created_before()`
+would change. In this example, it would just return `test_page`.
+
+These lists of dependent pages could just be concatenated for every `match_...()` function in a pagespec - you can ignore
+the boolean formula aspects of the pagespec for this. If a content dependency were added on these pages, then I think
+the correct rebuilds would occur.
+
+In all, this is a surprisingly difficult problem to solve perfectly. Consider the following case:
+
+PageA.mdwn:
+
+> [ShavesSelf]
+
+PageB.mdwn
+
+> Doesn't shave self.
+
+ShavedByBob.mdwn:
+
+> [!include pages="!link(ShavesSelf)"]
+
+Does ShavedByBob.mdwn include itself?
+
+(Yeah - in IkiWiki currently links are included by include, but the idea holds. I had a good example a while back, but I can't think of it right now.)
+
+sigh.
+
+-- [[Will]]
+
+> I have also been thinking about some sort of analysis pass over pagespecs
+> to determine what metadata, pages, etc they depend on. It is indeed
+> tricky to do. Even if it's just limited to returning a list of pages
+> as you suggest.
+>
+> Consider: For a `*` glob, it has to return a list of all pages
+> in the wiki. Which is expensive. And what if the pagespec is
+> something like `* and backlink(index)`? Without analyising the
+> boolean relationship between terms, the returned list
+> will have many more items in it than it should. Or do we not make
+> globs return their matches? (If so we have to deal with those
+> with one of the other methods disucssed.) --[[Joey]]
+