From what follows about SelectorModels, I'm presuming this is *not* a way to handle the multiple transformations I was describing in my previous point (focal plane coords, etc), but instead a way to handle situations where the pixel<>sky transformation have different properties at different points in the pixel array. I'm worried that the API seems to call this out as a special case. In general, special cases are "bad"... We've been doing this sort of thing in AST for a long time, and our experience is that it can be supported by the same API that is used by all other types of transformations. A SelectorMap is just like any other Mapping in the way it behaves and is used within the larger WCS framework.