            |      \             /      |               |
|               |     / \           / \     |               |
|               |    /   \         /   \    |               |
|               |   /     \       /     \   |               |
|               |  /       \     /       \  |               |
|               | /         \   /         \ |               |
|               |/           \ /           \|               |
|       (c, r)  o            (c)            o    r          |
|        c (r)   \  