"His method, which you can read about in more detail on his website (pdf), is a hierarchical structure that doesn’t seem entirely dissimilar from the two-column

"His method, which you can read about in more detail on his website (pdf), is a hierarchical structure that doesn’t seem entirely dissimilar from the two-column proofs that most of us learned in middle school or high school geometry class, although he points out that it can handle complex problems that would be unwieldy in that two-column format. Each line is numbered, and each assertion is justified with numbers referring to previous lines and assertions." This is just too obvious. Every proof has this structure, though sometimes this structure is left implicit (which may or may not be a good ting).