| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | at [adv] a | 1388 |
|
| 2 | at [adv] a | 1376 |
|
| 3 | at once [art] | 170 |
|
| 4 | [prep] once a | 99 |
|
| 5 | at once a | 65 |
|
