[andrewem]

A suggestion on your sample. It says “The librarian goes to the stacks, finds the book, and brings it back to you.

The next time you need that same book, the librarian doesn't have to go all the way to the stacks again – they can just grab it from the shelf behind the counter.”

This is missing the step where after you return the book to the librarian, they put it on the shelf behind the counter.

Then if you want you can talk about the shelf only fitting a limited number of books, and the librarian having to decide what to do if the shelf is full.

[cristianMD7]

This is very good suggestion, thanks. I'm in the process of rewriting some sections of the sample and will update the 'librarian' part.