Revert operation #2
Description
This should follow GIT semantics: given a change set C (or a list thereof), create and commit a new change set that has the effect of undoing all changes enacted by C. This way, history is preserved, but the state of the ontology gets reverted to what it was before C. Such an approach will make it easy to undo C, even if there were commits after it, which is useful when those commits don't conflict.