The control feature of `John' and the two PRO's all get co-indexed. This treatment might appear to lead to a problem. Consider ((61)):
If both the `want' trees have the control feature of their subject co-indexed to their foot nodes, we would have a situtation where the PRO is co-indexed for control feature with `John', as well as with `Mary'. Note that the higher `want' in ((60)) is wantECM - it assigns case to the subject of the lower clause while the lower `want' in ((60)) is not. Subject control is restricted to non-ECM (Exceptional Case Marking) verbs that take infinitival complements. Since the two `want's in ((60)) are different with respect to their control (and other) properties, the control feature of PRO stops at `Mary' and is not transmitted to the higher clause.