Suggestion: test for "BG-to-OBJ-to-OBJ" priority

[ISSOtm]

As documented in Pan Docs; it appears no test ROM checks for this. I noticed that from an Emulicious regression, of all things.

My understanding is that it may be questioned whether to add this to acid2, or put it with some more "advanced" tests into a separate "acid3"? (The former should add the ROM revision on-screen, IMO.)