See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/31593969 Handbook of Formal Languages Article · January 1997 DOI: 10.1007/978-3-642-59126-6 · Source: OAI CITATIONS 1,167 READS 440 2 authors: Grzegorz Rozenberg Leiden University 844 PUBLICATIONS 16,469 CITATIONS SEE PROFILE Arto Salomaa University of Turku 531 PUBLICATIONS 11,132 CITATIONS SEE PROFILE All content following this page was uploaded by Arto Salomaa on 03 February 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.