a "call" parameter, which is a reference to a function to call for the
hook.
+An optional "scan" parameter, if set to a true value, makes the hook be
+called during the preliminary scan that ikiwiki makes of updated pages,
+before begining to render pages. This parameter should be set to true if
+the hook modifies data in `%links`. Note that doing so will make the hook
+be run twice per page build, so avoid doing it for expensive hooks.
+
## Types of hooks
In roughly the order they are called.
make arbitrary changes. The function is passed named parameters `page` and
`content` and should return the filtered content.
+### scan
+
+ hook(type => "scan", id => "foo", call => \&scan);
+
+This is identical to a preprocess hook (see below), except that it is
+called in the initial pass that scans pages for data that will be used in
+later passes. Scan hooks are the only hook that should modify
+
### preprocess
Adding a [[PreProcessorDirective]] is probably the most common use of a