+ IkiWiki::hook(type => "preprocess", id => "foo", call => \&preprocess);
+
+Replace "foo" with the command name that will be used inside brackets for
+the preprocessor directive.
+
+Each time the directive is processed, the referenced function (`preprocess`
+in the example above) is called, and is passed named parameters. A "page"
+parameter gives the name of the page that embedded the preprocessor
+directive, while a "destpage" parameter gices the name of the page the
+content is going to (different for inlined pages). All parameters included
+in the directive are included as named parameters as well. Whatever the
+function returns goes onto the page in place of the directive.
+
+Note that if the [[htmlscrubber]] is enabled, html in
+[[PreProcessorDirective]] output is sanitised, which may limit what your
+plugin can do. Also, the rest of the page content is not in html format at
+preprocessor time. Text output by a preprocessor directive will be
+linkified and passed through markdown (or whatever engine is used to htmlize
+the page) along with the rest of the page.
+
+### htmlize
+
+ IkiWiki::hook(type => "htmlize", id => "ext", call => \&htmlize);