Chapter 15 mention "call stack" but never intruduce the concept

[lbellomo]

On Chapter 15 mentions "call stack" but it's never actually introduced (I don't recall seeing it in what I've read so far, and I can't find it with grep either). Maybe it deserves a better explanation or a link to something that explains it a bit.

[BartMassey]

Maybe. Much of the text may be inaccessible to folks who don't understand how the Rust stack works, but I'm open to suggestions for a sentence or two.