Function viewed 

pub fn viewed<'a>(
    doc: &'a Doc,
    time_machine: Option<&'a TimeMachine>,
) -> &'a Repo

The repo the canvas reads: the document’s own, or the prefix fold the time machine is holding. A free function over the two fields rather than a method, so the passes that also need &mut on the index, the presentation, or the gesture can still split the borrow.