I don’t think they suggested avoiding the checks, unless I’ve missed something?

The part that's confusing me is the part about avoiding clutter – if you have to do all the checks anyway, what clutter are you avoiding by changing the order of them?

I believe the likelihood annotations are the things they are talking about, for cluttering the code. Not the argument checks.

That conditional would be checked, then if it failed, checked again after the normal case has run, in order to choose the arm to follow.

Not a huge deal execution-time-wise, but from a reader’s PoV, the way I write it says, “ok, the special cases don’t apply to the body so I don’t have to worry that the index will be out of range (or whatever) and can just focus on the logic”.

Maybe the implied question is that, the compiler can optimise the checks to the occur in whatever order it wants.