+------+------+---------+
| a    | b    | control |
+------+------+---------+
| a    | a    | aa      |
| a    | b    | ab      |
| a    | c    | ac      |
| b    | a    | ba      |
| b    | b    | bb      |
| c    | a    | ca      |
| c    | b    | cb      |
| c    | c    | cc      |
+------+------+---------+