| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | [prep] once a | 87 |
|
| 2 | at once a | 65 |
|
| 3 | to once a | 14 |
|
| 4 | [prep] once an | 12 |
|
| 5 | for once a | 8 |
|
