In my view without that simple defining rule it is difficult to come up with a proposal that is simple,

I think I have decided that it was not really closed world I was thinking of, but more along the lines of what Simon PJ posted... After all at some point the compiler _has_ to commit to an instance. If we leave everything as it is - unless the compiler cannot choose an instance due to unresolved overloading. In which case my suggestion is that the compiler trys unifying all instances with the environment, and if one and only one instance unifies then use that one... Keean.