1. The algebraic theory of context-free languages;Chomsky,1963
2. Hierarchy of indexed languages of arbitrary level;Maslov;Sov. Math. Dokl.,1974
3. Indexed grammars–an extension of context-free grammars;Aho;J. ACM,1968
4. Higher-order pushdown trees are easy;Knapik,2002
5. Collapsible pushdown automata and recursion schemes;Hague,2008