Support for len() for a ProductSet

> I understand that the cardinality of an Interval set or the product of an Interval set with any other set is not correctly implemented by len().

Yes.  Then just remove this method from the Interval.

> However, is the cardinality defined by len() for a product set of finite sets? I think so. 

Ok.  I'm convinced to have this method for any generic set classes, including PowerSet.