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-1 on that, Greg.  If Python sees

    list + set

it's got no more reason to believe that the programmer 
intended the list elements to be inserted into the set than 
to believe the intent was to append the set elements to the 
list.  "Be generous in what you accept" comes from the 
networking world, where the chances are good that the 
program on the other end was written by people who can't 
code confused by an ambiguous spec written by people 
who can't write <0.9 wink>.