1. Side effects may be permanently destructive, making Undo infeasible. How do you undo emptying the recycle bin? How do you undo posting a cert on the public i

1. Side effects may be permanently destructive, making Undo infeasible. How do you undo emptying the recycle bin? How do you undo posting a cert on the public internet? 2. Cancel is not just for “are you sure”. It’s often there for “never mind, let me go back”, as in the file open dialog example.