but what's the type of update?
if you have b : Board I (where I is the index set) then to do update(b,i,v) you know you want i : I, but what's I' in the result type Board I'? sure seems like it wants to be I - i ?
@chrisamaphonechris martens yes it is I - i! i spent way too long relearning the utilities but here's an impl in typescript: Playground Link. it's a not-so-uncommon pattern when typing HTML elements.
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