Undo

Allow an operation to be reverted so that user-specific undo can be supported. For example:
1. User 1 types "abc"
2. User 2 types "def"
3. User 1 clicks undo

An operation reverting the first change would need to be generated so that "abc" is deleted and not "def", even though "def" was last typed into the document. In other words, op1 = { insert "abc" }; revert(op1) = { delete 3, skip 3 }.
