if heap[i].ev.time == 5 {
			heap[i].ev.time = -21
			heapUpdate(heap, i)
			break
		}
	}
	checkHeap(t, heap)
	if heap[0].ev.time != -21 {
		t.Fatalf("heap update failed, expected %d as heap min: %s", -21, heapDebugString(heap))
	}

	// Update the minimum element to be smaller. There should be no change.
	heap[0].ev.time = -22
	heapUpdate(heap, 0)
	checkHeap(t, heap)
	if heap[0].ev.time != -22 {

	// Add the cursor to the end of the heap.
	heap = append(heap, bc)

	// Sift the new entry up to the right place.
	heapSiftUp(heap, len(heap)-1)
	return heap
}

func heapUpdate(heap []*batchCursor, i int) {
	// Try to sift up.
	if heapSiftUp(heap, i) != i {
		return
	}
	// Try to sift down, if sifting up failed.
	heapSiftDown(heap, i)
}

		// Refresh the cursor's event.
		ok, err := bc.nextEvent(r.gen.batches[bc.m], r.gen.freq)
		if err != nil {
			return false, err
		}
		if ok {
			// If we successfully refreshed, update the heap.
			heapUpdate(r.frontier, i)
		} else {
			// There's nothing else to read. Delete this cursor from the frontier.
			r.frontier = heapRemove(r.frontier, i)
		}
		return true, nil
	}