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Hi,
i have two sets s1 and s2. i want to produce a new set s, that is the union of s1,s2. it would make sense to use set_union(s1.begin(),s1.end(),s2.begin(),s2.end(),...)
whats the 5th parameter should be?

This should be iterator to the beginning of the storage where union of these two sets will be placed to. See your favourite STL vendor documentation for more details (or alternatively try this: http://www.sgi.com/tech/stl/)