fn update_meet<T: Lattice+Clone>(meet: &mut Option<T>, other: Option<&T>) {
        if let Some(time) = other {
            if let Some(meet) = meet.as_mut() {
                *meet = meet.meet(time);
            }
            if meet.is_none() {
                *meet = Some(time.clone());
            }
        }
    }
Updates an optional meet by an optional time.
differential-dataflow/src/operators/reduce.rs