These expectation maximization (EM) experiments were motivated by the desire to overcome the limitations imposed by the heuristics that have been heretofore used to perform head-lexicalization in treebanks.
with arguments identified as described in Section 4.9, if a subjectless sentence is found to have an argument prior to its head, this step detransforms the SG so that it reverts to being an S. Head-finding is discussed at length in Collins' thesis, and the head-finding rules used are included in his Appendix A.