This is in the part of the code which persists to split; so, xblock.location will be a locator. The use of `location` here is to ensure there's a map from every locator to a location; so, the real question is why is this code still adding maps. I think I had a reason but obviously failed to document it in a manner I can find. I'll need to check that. My gut is that the only things which need maps are things which were moved from old mongo to split. Arrrrgggghhhh