Criteria to Disprove Context-Freeness of Collage Languages

Collage grammars generate picture languages in a context-free way. The generating process is based on the replacement of atomic nonterminal items and can be seen as an adaptation of the notion of hyperedge replacement known from the area of context-free graph generation. While a pumping lemma holds for hyperedge replacement graph grammars and is quite useful to show that certain graph languages cannot be generated, the same technique fails in the collage case, unfortunately. But, in this paper, we present some other criteria that allow to disprove context freeness of collage languages.