Bag context is a device for regulated rewriting in tree and string grammars. It represents context that is not part of the developing tree or string, but evolves on its own during a derivation. Motivation for investigating bag context tree languages is provided by showing that the class of bag context tree languages is the closure of the class of random context tree languages under linear top-down tree transductions. Furthermore, we establish an interchange theorem for subtrees of dense trees generated by bag context tree grammars. This result implies that the class of bag context tree languages is incomparable with the class of branching synchronization tree languages
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