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I wonder if the binary operators perhaps should be changed to 
allow other to be a sequence (or really, anything iterable).  
That is, if other is not a set type, try to create an 
ImmutableSet using other and, if successful, use that for the 
operation. Of course, the conversion of iterable to set can be 
done explicitly by the code which uses the set operation, but 
it might be desirable to make this conversion implicit (under 
the "be generous in what you accept" principle).