The "Closed-class Constraint", developed by Aravind Joshi, posits that closed class items (pronouns, prepositions, conjunctions, etc.) cannot be switched. The "Functional Head Constraint" developed by Belazi et al. holds that code-switching cannot occur between a functional head (a complementizer, a determiner, an inflection, etc.) and its complement (sentence, noun-phrase, verb-phrase). These constraints, among others like the Matrix Language-Frame model, are controversial among linguists positing alternative theories, as they are seen to claim universality and make general predictions based upon specific presumptions about the nature of syntax.