We are gratefully indebted to the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) for its continuated support to the development of this language through the projects ACRE (CAYCIT 836/86), SPES (880J382), ARREL (TIC92-0579-C02-01) and SMASH (TIC96-1038-C04-01). This research has also been supported by Esprit Basic Research Action number 3085 (DRUMS) and by the European Community project MUM (Copernicus 10053). The definition of this language has profited from ideas coming from many people, among which we would like to thank Ramón López de Mántaras, Lluís Godo, Francesc Esteva, Jaume Agustí and Don Sannella. Our thanks to the people that have developed applications based on Milord II, Albert Verdaguer, Miquel Belmonte, Marta Domingo, Pilar Barrufet, Lluís Murgui and Ferran Sanz.
The final version of the paper has been produced while Carles Sierra was on sabbatical leave at the Queen Mary and Westfield College, University of London, thanks to the Spanish Ministry of Education grant PR95-313.