1. An approach to a unified theory of automata;Hopcroft;BSTJ,1967
2. Abstract families of languages, in Studies in Abstract Families of Languages;Ginsburg;AMS Memoir No. 87,1969
3. Abstract families of deterministic languages;Chandler,1969
4. Two-way balloon automata and AFL;Ginsburg;JACM,1970
5. Two way pushdown automata;Gray;Inf. Control,1967