If the compiler treated instances as closed in this way, then adding a new instance to the program could break existing parts of the program.

But there are many ways to do this... and besides this doesn't really make sense... consider the following: module A defines class X module B imports class X defines some instances module C imports A & B Now, you can add any instance you like to module C and it is never seen by modules A and B... In other words even if you assume closed classes you cannot break an existing module without editing that module or one imported by it (in which case you can break _anything_ by deleting a function) If you don't allow overlapping instances none of this makes any difference anyway - closed or open ... (instance selection cannot change if no instances are allowed to overlap) Finally if you allow overlapping instances you can break existing code (without the closed assumption) just by putting in a more specific instance in a module included in some module using the more general instance. Thoughts? What is the problem with assuming all classes are closed within the available context (imported modules) Keean.