There are three ways to cancel now:
1. Save an empty twt.
2. Quit the editor without ever saving anything to the file.
3. Quit the editor with an exit code other than 0 (":cq!" in Vim).
I think this is good enough. I won’t add anything that works on the twt’s content (i.e. “same content = cancel”), because I want to be able to write twts with arbitrary content (even if it happens to be identical to that pre-filled content).=