This is a jquery plugin that allows you to easily add undo/redo
functionality to your application.

It uses jQuery's $.data store to keep a stack of undo and redo
mementos.  A memento is any object which has both 'undo' and 'redo'
functions.  An optional 'commit' function will be called, which can
make implementation easier.

The most common usage is to create an object that knows how to handle
a specific type of state change.  Whenever we change this state in our
application, we create a new memento of this type, and tell the
history plugin to remember it.

function ChangeFooMemento(foo) {
  var before, after;
  before = foo.dumpState();
  
  function commit() {
    after = foo.dumpState();
  }

  function undo() {
    foo.restoreState(before);
  }

  function after() {
    foo.restoreState(after);
  }
}

$(bar).history(); // create the history

// ... later

var memento = new ChangeFooState(foo);
foo.change();

// We now tell the history to remember the state.  This will call
// foo's commit, and put it on the undo stack.  Also, the redo stack
// is cleared here.
$(bar).history('remember', foo);

// ... later

// Next we call undo.  foo's state is returned to what it was before
// foo.change() (so long as dumpState and restoreState work).  undo
// will pop the memento off the undo stack, and push it on the redo
// stack.
$(bar).history('undo');

// ... later

// Finally we can redo.  The memento is popped off the redo stack, and
// pushed on the undo stack.
$(bar).history('redo');