| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | difficult [to-vb] [prep] [art] | 443 |
|
| 2 | difficult [to-vb] [prep] [art] | 443 |
|
| 3 | [adj] [to-vb] at [art] | 439 |
|
| 4 | difficult [to-vb] at [art] | 17 |
|
| 5 | difficult [to-vb] at [art] | 17 |
|
| 6 | difficult [verb] at [art] | 17 |
|
