In Martin-Löf type theory, the elimination form for identity types seems to always be called J. Is this short for something? Is it just because it's the letter after I? I can't find anything explaining why.
1 Answer
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Yes, that is the most likely explanation (in fact the only one I'm aware of). See also Remark 4.2.3 in Principles of Dependent Type Theory.
Axiom K was then named for the next letter after J, and axiom L... you get the picture.