We study the set of possible configurations for a general kinetically constrained model (KCM), a non monotone version of the U-bootstrap percolation cellular automata. We solve a combinatorial question that is a generalization of a problem addressed by Chung, Diaconis and Graham in 2001 for a specific one–dimensional KCM, the East model. Since the general models we consider are in any dimension and lack the oriented character of the East dynamics, we have to follow a completely different route than the one taken by Chung, Diaconis and Graham. Our combinatorial result is used by Marêché, Martinelli and Toninelli to complete the proof of a conjecture put forward by Morris.