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...