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",
+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