heap example

if (!$this->waitUntil
            || $this->waitUntil > ($now = $this->clock->now())
            || !$checkpoint->acquire($now)
        ) {
            return;
        }

        $startTime = $checkpoint->from();
        $lastTime = $checkpoint->time();
        $lastIndex = $checkpoint->index();
        $heap = $this->heap($lastTime, $startTime);

        while (!$heap->isEmpty() && $heap->top()[0] <= $now) {
            /** @var \DateTimeImmutable $time */
            /** @var int $index */
            /** @var RecurringMessage $recurringMessage */
            [$time, $index, $recurringMessage] = $heap->extract();
            $id = $recurringMessage->getId();
            $trigger = $recurringMessage->getTrigger();
            $yield = true;

            if ($time < $lastTime) {