Function classify_time 

fn classify_time<T: PartialOrder>(
    persist_upper: &Antichain<T>,
    ts: &T,
) -> TimeClass

Classify ts against persist_upper: the single spelling of the drain’s eligibility test. The chunked drain’s probe-collection pass and its classification pass, and the paged drain’s cursor walk, all call this, so the probe set and the classification cannot disagree. Under the operator’s total order, Eligible means ts equals the frontier’s one element.