Hello, I have recently tried to implement merging of more lists (or union of more sets). The (somewhat silly) approach would be set<X> ret,ret2; for(list<set<X> >::iterator it=sets.begin(),end=sets.end();it!=end;++it) { set_union(ret.begin(),ret.end(),it->begin(),it->end(),inserter(ret2)); ret.clear(); swap(ret,ret2); } and ret is the result. However, it seems to me that these temporaries are unnecessary. So I wonder wheter there could/should be some class of iterators that implemented some STL algorithms. There is already an iterator that could replace the unary version of transform (transform_iterator) In my case, I'm talking about some merge_iterator that would merge two sorted sequences (being an input iterator). That is, copy(merger(a.begin(),a.end(),b.begin(),b.end()),ostream_iterator(cout)) would be equivalent to merge(a.begin(),a.end(),b.begin(),b.end(),ostream_iterator(cout)) and similarly for set_union etc. Is it possible to have such iterators? Sincirely Jiri Palecek