List of empty categories: traces and other null elements.
| IPCHG/PPCHE/HeliPaD | use |
|---|---|
| 0 | empty operator (most often C or WNP) |
| *con* | subject elided under conjunction, identical to subject of first conjunct |
| *pro* | other null referential subject |
| *exp* | empty expletive subject |
| *T* | trace of wh-movement (questions and relative clauses) |
| *ICH* | "interpret constituent here": trace of other movement (often extraposition) |