Tree transducers, L systems and two-way machines (Extended Abstract)

This extended abstract is a condensed version of the results presented in two technical reports ([16] and [13]). In [16] a systematic treatment of the relationships between parallel rewriting systems (top-down tree transducer, ETOL system) and two-way machines (2-way gsm, tree-walking automaton, checking stack automaton) is given. Particular attention is paid to the effect of restricting the copying power of these devices. In [13] the results of [16] are employed to show that the iteration of nondeterministic top-down tree transducers, of nondeterministic 2-way gsm's and of control on ETOL systems each gives rise to a proper hierarchy.