Hi Alex,
Glad to hear you are finding the book useful. This one of a handful
of proofs that are very difficult to chase your way through, and it
comes early in the book.
Zoltán has the right explanation. What should I put out to the right
as a reason to make this clear? "Just above", "Previous ____" or
something else? Suggestions would be welcome.
I think the third and fourth steps need the same treatment, and I see
now that the very end should appeal to Definition ME also.
Rob