Found that it was too hard to implement Scala's 'mutable.Set' trait, even 'Traversable' was too hard, due to the complex web that implementation Scala's collections.  Instead, have created my own 'UnorderedSequentialSet' trait which implements as many 'mutable.Set' traits as is practical and appropriate.

The code compiles, but 'SesameTripleSet' lacks method implementations, and all of the test code is currently commented out.  As I implement the methods, I'll restore and add tests.