Idea: Once-only diverts and tunnels

(Originally from an internal discussion between @joningold and me on our internal Asana.)

Basic syntax: `->? A` means "go to A if you've never been there before."

Could also use it on tunnels: `->? A ->`

And chain them: `->? A -> B ->? C ->` means "go to A if you've never been there before, but either way go to B next, then C if you've never been there before".

Looks useful, but is it unnecessarily extra syntax...?