On 0L-Languages

Summary In 0 L -languages, words are produced from each other by the simultaneous transition of all letters according to a set of production rules; the context is ignored. (i) 0 L -languages are not closed under the operations usually considered. (ii) 0 L -languages over a one-letter alphabet are discussed separately; a characterization is given of a subclass. (iii) 0 L -languages are incomparable with regular sets, incomparable with context-free languages, and strictly included in context-sensitive languages.