hook(type => "filter", id => "foo", call => \&filter);
-Runs on the raw source of a page, before anything else touches it, and can
-make arbitrary changes. The function is passed named parameters "page",
-"destpage", "content" and "fullpage". "fullpage" is a true value if,
-and only if, a full page's content is being filtered, e.g. as opposed
-to a directive parameter. It should return the filtered content.
+Runs on the full raw source of a page, before anything else touches it, and
+can make arbitrary changes. The function is passed named parameters "page",
+"destpage", and "content". It should return the filtered content.
### preprocess