It's normal for users to explore some part of the program execution, reach a dead end, and then want to back up to a previously explored point and continue in a new direction. Pernosco integrates with the browser history for this purpose. In particular the browser back button will undo the last change in focus, and then the forward button will redo it. See an example.
The browser's current URL always encodes the current focus. Users can save or exchange URLs to point to particular moments of interest. Here's an example: