Thanks for the report, Matthias.

> I can talk to Thomas Kluyver tomorrow about merging green tree snake into the devguide,

devguide isn't the correct place for such documentation.

> or maybe into ast docs. Would that be useful/more acceptable than linking to external docs ?

There is an open issue for that: issue 16544. I've talked to Thomas about merging greentreesnakes to the AST documentation last year and already have a WIP patch at https://gist.github.com/berkerpeksag/0be9bb0ada548185d9ef (I have more complete patch, but couldn't find it now :))

If you have time to work on it, I can upload it to the tracker item and we can work together.