Ch19 Poor example for why we should use `let` instead of `set`

[Rich]

Chapter 19:

Why would we want to do this when we could just use set? Run the following
commands:

:let &textwidth = &textwidth + 10
:set textwidth?

This time Vim displays "textwidth=110". When you set an option using set you
can only set it to a single literal value.

But in this case we can also do exactly the same thing with set:

:set textwidth=100
:set textwidth+=10
:set textwidth?

Vim displays "textwidth=110".

[Steve Losh]

Yeah, it's tough to come up with simple examples that don't require a lot of outside knowledge. If you can think of a good standalone example here let me know.

[Filipe Brandenburger]

One possible example would be a command to increase textwidth by 15% or something similar.

:let &textwidth = &textwidth * 115 / 100

That couldn't really be accomplished using :set (well there's ^= but that doesn't seem to work with a float.) Just a suggestion... Thanks for the excellent guide!!!