As far as I understand Prospero has implemented the "prepare ⇒ apply" sequence of operations for both an update and a revert, with the prepare operation allowing the user to check the pending changes before proceeding. The installation-manager CLI on the other hand allows this only for update, while "installer revert" prepares a candidate straightaway with no confirmation from the user and no list of changes. HAL uses "history-from-revision" to display changes but that's not the same thing.