Ok, other computer diagnosis was the 2015 MacBook Air which had been running Big Sur beta 3 when it stopped booting the other day.
Even when plugged in and the little magsafe light is on, it doesn't respond at all to pressing the power button. No boot or attempted boot.
I think I'm just going to retire it, it could be a mainboard problem and that's hard to replace for an obsolete machine.
I often recall a bit from the book 3001 (sequel to 2001, 2010, and 2061) where the revived 2001-era Frank is disappointed that spacecraft techs in the year 3001 don't really have any way to diagnose or repair circuits: they can only identify which cards are defective and swap them out for working ones.
Such is the life of integrated circuits...
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