tree automaton

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tree automaton A generalization of the notion of a finite-state automaton, applying to trees rather than strings (see tree language). There are two versions. A top-down machine begins at the root of the tree; having read the symbol at a node it changes state accordingly and splits into n machines to process separately the n descendants. A bottom-up machine begins with several separate activations of itself – one at each leaf node of the tree. Whenever all the subtrees of a particular node have been processed, the machines that have processed them are replaced by a single one at that node. Its state is determined by the symbol at the node and the final states of the descendant machines.