* Bulletproof the method for detecting mutations during iteration.   The previous approach was too easily fooled (a rotate() sufficed).
* Bulletproof the method for detecting mutations during iteration.
  The previous approach was too easily fooled (a rotate() sufficed).

* Use it->counter to determine when iteration is complete.  The
  previous approach was too complex.

* Strengthen an assertion and add a comment here or there.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator