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Or, even better, also from the cookbook: {(2#x)#1,x#0} But this really borders on obfuscation :P

That does require someone to know that the take operator continues to treat the y list as circular when x is a list.

I think this form might be a bit easier: {(x,x)#(x*x)#1,x#0}

But then you're not flexing your q muscles. Btw this is from the cookbook, these obfuscated patterns are actually recommended! https://code.kx.com/phrases/matrix/#identity-matrix-of-order...