I am reading a textbook on computer science, and now the author explains a proof of existence of non-recursively enumerable language (he calls it $L_{diag}$) by diagonalization method, that is very reminiscent of Cantor's diagonalization and is the same in principle as this one - http://www.pmshiva.elementfx.com/cs530/rep2/G17.ppt .
So far so good.
However, then he asks a question as an excercise: "Generalize diagonalization by using permutations." And I have no clue what he means by that.