Now it wants the u,v fields on the tracer points. I'd been told that, but hadn't thought that someone would give me u,v on u,v points when they knew that. The tracer sponges use
horiz_interp_and_extrap_tracer_record and now the u,v sponges do too. Maybe it's time for changes to the sponge code if the sponges are provided on the native grid.