std::set<> internally is often a red-black tree which is fairly
expensive to create but fast to lookup. In the case of
sort_and_unify(), a set<> is constructed as a temporary object to
attempt to speed up lookups. Being a temporarily, however, the cost of
creation far outweights the lookup improvement and is a net performance
loss. Instead, sort the vector<> that already exists and then apply
std::unique().
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