| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | must have been [v-ing] [prep] [pron] | 18 |
|
| 2 | must have been [v-ing] [prep] [pron] | 18 |
|
| 3 | must have been [v-ing] [prep] [pers pn] | 9 |
|
| 4 | must have been [v-ing] [prep] [pos pn] | 7 |
|
