+--------------+-----------------------------------+
|  High-level  |             Low-level             |
+----+----+----+------+------+---------------------+
| ca | cb | cc |  ca  |  cb  | cc = 1/8 + ca + cb  |
+----+----+----+------+------+-------------+-------+
| 0  | 0  | 0  | -1/8 | -1/8 | -1/8 ~= 7/8 | 0.875 |
+----+----+----+------+------+-------------+-------+
| 0  | 1  | 1  | -1/8 | 1/8  | 1/8         | 0.125 |
+----+----+----+------+------+-------------+-------+
| 1  | 0  | 1  | 1/8  | -1/8 | 1/8         | 0.125 |
+----+----+----+------+------+-------------+-------+
| 1  | 1  | 1  | 1/8  | 1/8  | 3/8         | 0.375 |
+----+----+----+------+------+-------------+-------+