The rule is that there is no rule. This is not like there is an Academy or another governing body that governs what people get to name their programming language and how they should write it. Each of them makes its own rules for its own language.
As a rule, if you write about this material, you either follow the style guide for the house, or see the official name for it.
The reason you see so many different uses is because most people do not care or are simply ignorant. I still wonder who ever told anyone to refer to a Macintosh as "MAC", but this spelling is common. Some people just love their toggle keys, I think.
brian d foy
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