Hi, Below is an assumption (which could be wrong) and two questions. ASSUMPTION 1 Only smaller models can be specified using the sub-class mechanism. For example if we directly make a subclass A => B then every instance of B must also be an instance of A (B implies A or B is a subset of A). Hence, type classes cannot be combined using sub-classing to provide specifications of bigger models. QUESTIONS 1) If assumption 1 is correct, is there a mechanism whereby the module system can be used to construct bigger models? 2) If there is such a mechanism does it involve type classes or is it a module only solution? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie