Function push_unit 

fn push_unit(
    engine: &mut Engine<'_>,
    report: &Reporter<'_>,
    unit: Pending,
    owed: Option<Deferred>,
) -> Push

Push one unit and record exactly one ledger entry against it.

The single place both happen, so DeferredEffects::len() == PendingQueue::len() holds by construction rather than by review.