I meant to cover these last time but decided not to so as to keep the videos to about 10 minutes each.
First up this time round is iedit - a really cool mode that lets you select all the instances of the marked selection at the same time and edit them all at once. By default it's bound to C-l.
For example, let's say you had the following code:
You could mark one instance of somevar, hit C-; and you'd be editing them all at once. When you're done, you hit C-; again and everything is un-selected.
There are other ways to modify multiple instances of a selection. You can always search and replace or use Magnar Sveen's really cool multiple cursors package and I'm sure there are others but I like iedit since it's small and simple. It also works for rectangular editing but I didn't cover that in the video. I'll probably demo it when I cover rectangular editing in general.
Iedit has limitations. For insance, since it works globally, it isn't perfect for renaming variables. If the above code snippet was part of a larger program with many other instances of somevar, iedit wouldn't, by itself, be as useful.
You could reach for a language specific refactoring package but for me, that's where narrowing comes in. You can mark a region and tell emcas to narrow-to-region which hides everything outside the region and temporarily (until you widen) treats the region as the whole document.
So, you could narrow to the section of code you want to edit and then use iedit from within.
I think the video makes the behaviour pretty clear.
Video series overview page: