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I personally don't see the need for this.  Basically
everything here can already be done if you are willing to
create a set first and then use the set's methods.  Yes,
there might be a performancce difference if you are using a
dictionary, but if this functionality is really needed for
dicts perhaps it should be added to dicts themselves.  I
just  don't see any special reason to add this since this is
all easily done enough using sets.

Perhaps a PEP is in order to get this in?