Stuck on "Reduced Row-Echelon Form is Unique" proof

[super...@gmail.com]

Hi!
First of all, I would like to say - Robert, Big Thanks for such a
great book !!! I am learning Algebra on my own, bough paper copy and
after doing several chapters, have to say that this is one of the best
math books I held in my hands.

Question:
I am stuck on one step in "Reduced Row-Echelon Form is Unique" proof.

In the latest edition(2.1.1) , page 34, Second part of Second Step,
how do you get scalar δ equal to zero ?

Thanks !
- Alex

[Zoltán Tóth]

The first part of the second step proves it.

[Rob Beezer]

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