Thanks to Pere Garcia for helpful discussions on specialisation rule. We also thanks Lluís Murgui for advising in the medical example. This research has been supported by the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) through the project SMASH (TIC96-1038). 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.