sentences:  PRO can either be controlled ("obligatory control") or uncontrolled ("optional control").The realization that PRO does not behave exactly like an R-Expression, an anaphor, or a pronoun (it is in fact, simultaneously an anaphor and a pronoun) led to the conclusion that it must be a category in and of itself.It can sometimes be bound, is sometimes co-referenced in the sentence, and does not fit into binding theory.