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Heh, reading the diff alone doesn't help me; there's no
helpful context.

While the example use case doesn't seem interesting, I
consider non-support of sub-class overriding the format()
method to be a bug.

I'll try and look at the patch more seriously soon, but I've
not managed to eek out many tuits lately, and this week is
as massively overbooked as any these days.