Z-Automata for Compact and Direct Representation of Unranked Tree Languages
Unranked tree languages are valuable in natural language processing for modelling dependency trees. We introduce a new type of automaton for unranked tree languages, called Z-automaton, that is tailored for this particular application. The Z-automaton offers a compact form of representation, and unlike the closely related notion of stepwise automata, does not require a binary encoding of its input. We establish an arc-factored normal form, and prove the membership problem of Z-automata in normal form to be in \( O \left( mn \right) \), where m is the size of the transition table of the Z-automaton and n is the size of the input tree.
We thank the reviewers for carefully reading the manuscript. In particular, we thank one reviewer who pointed out a flaw in the original version of Theorem 1.
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