I may be missing something, but I really need to flatten out the array here; `concat` won't do what I want.

The alternative implementation I came up with here when writing it was

    [ [ , , ] assoc-each ] { } make reverse

but I wasn't convinced that was better.  (Although, in retrospect, I'm building up a list for no reason with the `>alist` call, so the above is probably better, anyway.)