Languages Modulo Normalization

We propose a new class of tree automata, called tree automata with normalization (TAN). This framework extends equational tree automata, and improved the results of them: recognized tree languages modulo the idempotency f(x, x) = x are closed under complement, besides we do not lose an important decidability result. In the paper, first we investigate the closure properties of this class for Boolean operations and the decidability relative to the equational tree automata. Next we consider the relationship to other automata frameworks, in particular, hedge automata, which is a class of unranked tree automata. Hedge automata have been recognized in the XML database community as a theoretical basis for modeling the manipulation of semi-structured data. Through the observation about transformations from XML schema to tree automata, we discuss advantages in the expressiveness and complexity of TAN. As an application of our framework, we show an example of XML schema and constraints that can be dealt with by TAN.