Two-way balloon automata and AFL
Seymour Ginsburg, John E. Hopcroft · Journal of the ACM · 1970
It is shown that if the family of languages accepted by a closed class of two-way balloon automata is closed under length-preserving homomorphism, then this family is an abstract family of languages (AFL) closed under intersection and e-free substitution.It is then proved that the family of languages accepted by the closed class of (nonerasing) (deterministic) stack acceptors is such a family.