We built a complete new collection. The ABC definitions included a number of methods automatically. These inherited methods might not be particularly efficient, but they're defined, they work, and we didn't write the code for them.

>>> s1 = Tree(["Item 1", "Another", "Middle"]) 
>>> list(s1) 
['Another', 'Item 1', 'Middle'] 
>>> len(s1) 
3 
>>> s2 = Tree(["Another", "More", "Yet More"]) 
>>> 
>>> union= s1 | s2 
>>> list(union) 
['Another', 'Another', 'Item 1', 'Middle', 'More', 'Yet More'] 
>>> len(union) 
6 
>>> union.remove('Another') 
>>> list(union) 
['Another', 'Item 1', 'Middle', 'More', 'Yet More'] 

This example shows us that the set union operator for set objects works properly, even though we didn't provide code for it specifically. As this is a bag, items are duplicated properly, too.