Dictionaries, popitem() method example

[Antonios B. Kaklis]

Hello to everybody...
There is something that i' don't understand.
In the Python Library Reference, Section 2.1.6 "Mapping Types"
It says: "a.popitem( )" --->Note 6: Is useful to destructively iterate over
                                                     a dictionary as often
used in set algorithms!

I don't understand  1) what is "Destructively iteration over a dictionary"
                             2) Set Algotithms
Any examples?
Thanks

Antonis
Greece