Extract setfs into UPDATE-CONTENT-METADATA.
PAGE-URL should always return a pathname.
Ensure that sitemap includes all content.

I had introduced a bug by depending on sitemap being the last subclass
in the ALL-SUBCLASSES list. Since there's only one instance of sitemap,
it's not *too* hackish to do everything in the publish step.
Sketch out incremental plugin.
Push sketch slightly further up hill.
Flesh out the sketch a bit more and add some constraints.