Function answered_later 

fn answered_later(messages: &[Message], from: usize, open: &[String]) -> bool

Whether any of open is answered later than from.

The settled half of the delivery rule’s bound: a transcript keeps a call whose result never arrived - the restart shape (#1836) - and a row after it belongs to the next turn. Asked on demand, at the rare row that needs it, so a fold over a settled conversation pays a scan only where it joins; the live walk never asks, because the caller’s flag answers first.