| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | must have been [v-ing] a [noun] | 5 |
|
| 2 | must [verb] been [v-ing] a [noun sg] | 5 |
|
| 3 | must [vb infin] been [v-ing] a [noun] | 5 |
|
| 4 | must have been [v-ing] a [noun] | 5 |
|
