+pagestate by the meta plugin. Also, there are many different types of
+metadata, that would need to be matched with the pagespecs somehow.
+
+Quick alternative: Make add_depends look at the pagespec. Ie, if it
+is a simple page name, or a glob, we know a presence dependency
+can be valid. If's more complex, convert the dependency from
+presence to full.
+
+There is a lot to dislike about this method. Its parsing of the pagespec,
+as currently implemented, does not let plugins add new types of pagespecs
+that only care about presence. Its pagespec parsing is also subject to
+false negatives (though these should be somewhat rare, and no false
+positives). Still, it does work, and it makes things like simple maps and
+pagecounts much more efficient.
+
+----
+
+### Link dependencies
+
+* `add_depends($page, $spec, links => 1, presence => 1)`
+ adds a links + presence dependency.
+* `refresh` only rebuilds a page with a links dependency if
+ pages matched by the pagespec gain or lose links. (What the link
+ actually points to may change independent of this, due to changes
+ elsewhere, without it firing.)
+* So, brokenlinks can fire whenever any links in any of the
+ pages it's tracking change, or when pages are added or
+ removed.
+* To determine if a pagespec is valid to be used with a links dependency,
+ use the same set that are valid for presence dependencies. But also
+ allow `backlinks()` to be used in it, since that matches pages
+ that the page links to, which is just what link dependencies are
+ triggered on.