| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | had [been] [prep] operation | 15 |
|
| 2 | [vb past] [been] [prep] operation | 15 |
|
| 3 | [vb past] been [prep] operation | 15 |
|
| 4 | [vb past] [been] in operation | 14 |
|
