fn.is_stable updated to search for stability in a more robust, and I think more correct way.  I think the best way would be to find the dirivative of fn and xfix, but this way was easy to implement.  Perhaps as the course goes on I can find a better way.
fn.is_stable updated to search for stability in a more robust, and I think more correct way.  I think the best way would be to find the dirivative of fn and xfix, but this way was easy to implement.  Perhaps as the course goes on I can find a better way.

parenthetical-e/ds