You are right, not yet documented, but I think that's the extent of it, apart from ${entity2}. I'll check.
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Again from memory, I think there's ${noun2}. These can be pretty useful in the right circumstances. For example, I've noticed that when I GET ALL and one of the items is scenery, it says something like "You can't get that." followed by "You get the apple." I'd prefer it to say, "You can't get the tree." followed by "You get the apple." I think I might change the system message to say "You can't get ${entity}."