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> In actuality there are four different, valid programs that can be produced by the above set of tactics: [...]

Nitpick, but only two of those are valid programs. Two of them use `a` without binding it.

Nitpick. But unused bindings is a warning, it doesn't create an invalid program

These are unbound usings.

Those two programs don't use a in the Nothing case, they have Just _ instead, i.e. a hole. Those are just as valid as any other program with a hole. You could fill the hole with undefined if you wanted to.

The article was edited in the interim. The error the ancestor posts describes did exist before (I saw it, too), but no longer.

As noted by Twisol, those holes previously contained `(fab a)`.

I'd delete my comment now that it's addressed, but 1) it's too late, and 2) I'm too happy about "unbound usings".

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