* The garbage collector tries to avoid tracking simple containers which

can't be part of a cycle. As of now, this is true for tuples and dicts

containing atomic types (such as ints, strings, etc.). Transitively, a dict

containing tuples of atomic types won't be tracked either. This helps bring

down the individual cost of each garbage collection, since it decreases the

number of objects to be considered and traversed by the collector.

To help diagnosing this optimization, a new function in the :mod:`gc`

module, :func:`is_tracked`, returns True if a given instance is tracked

by the garbage collector, False otherwise.

(Contributed by Antoine Pitrou; :issue:`4688`.)