Multipass automata and group word problems
  We introduce the notion of multipass automata as a generalization of pushdown automata and study the classes of languages accepted by such machines. The class of languages accepted by deterministic multipass automata is exactly the Boolean closure of the class of deterministic context-free languages while the class of languages accepted by nondeterministic multipass automata is exactly the class of poly-context-free languages, that is, languages which are the intersection of finitely many context-free languages. We illustrate the use of these automata by studying groups whose word problems are in the above classes. 
Tullio Ceccherini-Silberstein, Michel Coornaert, Francesca Fiorenzi,   Paul E. Schupp and Nicholas W.M. Touikan
math.GR cs.FL