That's what I was wondering, considering we have something like that for a live document in most IDEs (remembering changes character by character and the like).

That's what I was wondering, considering we have something like that for a live document in most IDEs (remembering changes character by character and the like). If a document needed to be played back each time to see what it was then it would suck, you would need a format that both represents the final product and also has all the history in one file.