In this document I'll try to explain how undo/redo works in Gazpacho

Every action that can be undone and redone is wrapped inside a Command. This
class has several interesting features:

- Its 'execute' method can be called as many times as you want and every time
  it is called the action is undone and redone alternatively. This means
  a command saves its state after being executed and prepare itself to perform
  the opposite action each time it is called. For this reason every command
  constructor usually needs a lot of arguments so it has enough information
  to be undone and redone every time its execute method is called.

- Every time the user perform an action a command is created and Gazpacho
  check if the last command executed was of the same kind. If so it then
  tries to collapse the actual command with the old one. Fox example, the user
  is editing the name of a widget. For every letter he/she changes a new
  CommandSetName is executed and collapsed with the previous one. So when
  the user undo this command all the letters are changed at once because they
  are seen as one single CommandSetName command. Not all the commands can be
  collapsed, that's what the 'unifies' method check. Also note that if there
  any redo action pending no collapsing is done.

Given this command design, each project has a command stack where all the
commands are being pushed as they are executed. There is also a pointer (an
integer index) which tell us what is the next redo command available.