If you've seen me speak at the Wolfram Tech conferences, or chatted with me by email or on any of the various Mathematica forums, you probably eventually get the message from me that folks at WRI pay attention to what you have to say. It's one of the things that I'm actually quite proud of my colleagues for...I see a bunch of them all over the place out in the public, mixing it up with our user base, and hearing what the community has to say about our product, and then discussing those things internally.

So for a company with that kind of attitude, you've got to know that we were impressed at the level of organized vigilance on the part of the community in letting us know that Multiple Undo Is Important. It's a sometimes difficult position we're in because there are lots of people that we'd really like to satisfy, and in many cases with concepts that are genuinely useful and interesting. But like any company, there are limits to our resources, and prioritization is necessary. And sometimes really good ideas might be prioritized downward for any number of reasons...sometimes because very difficult choices had to be made, and sometimes for less noble (and in retrospect, perhaps silly) reasons. But you can bet that when Stephen Wolfram was faced at almost every user event with one of you folks consistently reminding him of the need for multiple undo, that there would come a day when he would insist that I completely clear my plate and make multiple undo my very top priority.

So, thanks for that! It's been an interesting project with lots of challenges, and one which I'm very proud to have worked on. Now, for those who wish to indulge my tireless droning, here's a bit of info about the feature.

• Multiple undo is a better single undo than our single undo system ever was. One of my happy moments was, after integrating the feature into our development build, I went through all of our open single undo bugs. And, without intending to do so, I had fixed over dozen outstanding single undo bugs.
• One of the things I realized quickly as I was implementing multiple undo was that it's very confusing to realize where you're at in history. Did I leave undo in a state where the Input and Output cells correspond to one another, or are they subtly different? So I added a feature to make it eminently clear by dimming Output cells that don't match their Input counterparts. Furthermore, the dimming will only take effect if you make an edit that actually changes how things evaluate. For example, adding a comment or a non-lexical space to an Input cell will not dim the Output cell.
• Undo just wasn't so easy to implement in an environment which sits on top of a sophisticated typesetting system, is fully user-programmable, can have simultaneous streams of action which should be unwound from one another for undo purposes. So undo has highly efficient internal consistency checks. Should a bug or limitation occur that might create a state inconsistent with history, the system stops undo from continuing and prevents damage from being done to your data.
• By default, undo is limited only by memory, but it's highly memory efficient.
• Usually flipping Dynamic variable states is not undoable. However, you can make changes to a DynamicModule variable undoable by using the UndoTrackedVariables option. Also, each input field has its own independently tracked undo state which will keep track of changes which are produced programmatically or by hand.

And we're not finished. Unfortunately, I'm still aware of a few bugs in the system which I'll be working on. And I have a few plans for expanding functionality, including:

• Allow user-assigned memory limits to undo state (which is documented, but not quite done in the initial v10 release...sorry about that!)
• More memory efficiency, including sharing of objects which might show up multiple times in undo history.
• Better programmatic control of and access to undo state
• Allow efficient disk spooling of undo state to reduce memory. Which, incidentally, is almost all we need to support crash recovery of files.
• And, well, need I say it again? Whatever good ideas we hear from you, the community.

Enough words from me. Pipe up if you see any problems or have any suggestions concerning undo.