The context-free languages can be described as the languages that can be generated by a context-free grammar or recognized by a (nondeterministic) pushdown automaton.
The context-free languages are not closed under complementation, and the class co-CFL contains all languages whose complements are context-free.
Is there a model of computation (described as a grammar, automaton, generalized rewriting system, etc.) that precisely captures the co-CFLs? For example, is there a modification we can make to CFGs to have them accept precisely the co-CFLs, or some new type of automaton for them?