We’re lucky that this never bit us. For the most part, these rules
aren’t actually used; most code programs against ArrowCache and doesn’t
get specialized enough for these rules to fire.
Even if we did have code that could trigger this rule, the situations
where it would actually fire are slim. In order for the rule to
typecheck at all, both sides of the pair being passed through the arrow
must have exactly the same type. Of course, that would just make this
even more hellish to debug.
Rewrite rules are dangerous.
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