else if (peerTree_parent.left == this) return peerTree_parent.getIndexInParent() - 1;
else if (peerTree_parent.right == this) return peerTree_parent.getIndexInParent() + 1;
else throw new Error("we're not a child of our parent!");
else if (peerTree_parent.left == this) return peerTree_parent.getIndexInParent() - 1;
else if (peerTree_parent.right == this) return peerTree_parent.getIndexInParent() + 1;
else throw new Error("we're not a child of our parent!");