How is the undo tree used in Vim?

Question

This answer says:

Vim\'s undo/redo system is unbeatable. Type something, undo, type something else, and you can still get back the first thing you typed

Answer 1

I'm aware this question has been answered, but I thought I'd add an example.

Create a new file and type:

this is a line

undol will display the undo tree. At this point you haven't undone anything

:undol

number changes  when               saved
     1       1  14:50:36

now press ESC and modify the line to:

this is a old line

switch to normal mode and press u (undo), this should remove "old". If you check undol, at this point you still have only one branch.

now modify the line so it says:

this is a new line

Now :undol shows:

number changes  when               saved
     2       2  87 seconds ago
     3       2  3 seconds ago

You can switch to the first branch by typing

:u 2

this will move you to the end of the branch associated with number 2. You can move along this branch with g+ and g-. At this point g+ will do nothing (you are at the leaf). If you press g- “old" will be removed (you are traversing the first undo tree). That is if you remove “old” with g- and press g+ again, “old" will be redone.

If you type

:u 3

You will jump to the leaf of the second undo branch and it will read:

this is a new line