As promised, here's a preliminary callback patch against HEAD.
* finish first, then merge
* merge now, export the done bits
* merge now, don't export the interface yet
As this doesn't touch any existing parts I'd be inclined to merge, and not
on a branch, making the last one my preferred approach, but YMMV.
-- Nikodemus Schemer: "Buddha is small, clean, and serious."
Lispnik: "Buddha is big, has hairy armpits, and laughs."