this paper pg 159-160 th $10.$ In the proof where $A=\{a_1,a_2...\}$ a countable set with $\bar A=F$ and the sequence $\{x_n\}$ is taken such that $x_n=a_i$ when $n\in M_i.$ Upto this point is fine. It then says 'We thus obtain a subsequence $\{k_n\}$, say, of the sequence $\{n\}$ of positive integers' Which subsequence are they talking about $?$ And then the sequence $\{x_{k_n}\}$ and the proof is taken further. But what are the $k_n$'s ?
Thank you.