  Chomskyans contest that there are many other cases in which  speaker-hearers know grammatical rules, the critical evidence in  favor of which is missing from the pld. Kimball 1973:73-5,  for instance, argues that complex auxiliary sequences like might  have been are “vanishingly rare” in the  pld, hence that children acquire competence with these  constructions (in the sense of knowing the order in which to put the  modal, perfect and progressive elements) without relevant experience. (Pullum and Scholz 2002, discuss two other well known examples.)  Nativists thus conclude that numerous other principles of UG are  innately known as well. Together, these UG principles place strong  constraints on learners' linguistic theorizing, preventing them from  making errors for which there are no falsifying data.