Function open_turn 

fn open_turn(turns: &mut Vec<TurnSpan>, index: usize) -> usize

Open a turn, answering the span it opened: the one a page is cut on and the one a later report names.

One place opens a turn. The fold has two ways in - a block that is the user’s own turn, and a queued prompt hoisted into one - and a span pushed at only one of them leaves every turn of the other kind outside the page’s boundary list.