| http://www.w3.org/ns/prov#value | - Key() could return a std::pair, the first member is something indicating the class order (maybe a char such as 'b', 'c' and 'g', conveniently already in the right order), the second member indicating the rank/order within the class (this would be the id/var data member within the class). std::pair already supports the 2-level sort.
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