Undo-tree

I found undo-tree in Emacs .emacs.d folder, and I was confused about it at first. After I Google it and try the commands, I found it an awesome feature to maintain an undo or redo case.
There are five main usage of undo-tree together with prelude Emacs configuration shortcut.

uu      open the undo-tree mode
p or n  move the states up and down
b or f  switch at some branches
t       show the edit time tags
q       exit the undo-tree mode