public function scheduleForDelete(object $document) : void
    {
        $oid = spl_object_hash($document);

        if (isset($this->documentInsertions[$oid])) {
            if ($this->isInIdentityMap($document)) {
                $this->removeFromIdentityMap($document);
            }
            unset($this->documentInsertions[$oid]);
            return; // document has not been persisted yet, so nothing more to do.
        }

        if (! $this->isInIdentityMap($document)) {
            return; // ignore
        }

        $this->removeFromIdentityMap($document);
        $this->documentStates[$oid] = self::STATE_REMOVED;

        if (isset($this->documentUpdates[$oid])) {
            unset($this->documentUpdates[$oid]);
        }
        if (isset($this->documentDeletions[$oid])) {
            return;
        }

        $this->documentDeletions[$oid] = $document;
    }