def __iter__(self):
        """
        Iterate over keys with non-zero lengths.
        
        DS.__iter__(k) <==> for k in D 
        """
        for (key, val) in self.items():
            if len(val) > 0:
                yield key

        Iterate over keys with non-zero lengths.
        
        DS.__iter__(k) <==> for k in D 
        
recipes/Python/577702_DictSet__specialized_Pythcontainer_datatype/recipe-577702.py