      |
|                /     \ @ /     \                |
|               / x   y \ / x   y \               |
|              o  o---o  o  o---o  o              |
|             / \  \    / \    /  / \             |
|            /   \  @  /   \  @  /   \            |
|           /     \   /     \   /     \           |
|          /   y   \ /       \ /   y   \          |
|         o    @    o    @    o    o    o         |
|        / \       / \       / \   |   / \        |
|       /   \     /   \     /   \  @