#include <boost/range/algorithm.hpp>
#include <vector>
#include <numeric>
#include <cassert>

struct greater_by_two
{
    template <class T>
    bool operator()(const T& left, const T& right) const
    {
        return left / 2 == right / 2;
    }
};

int main()
{
    std::vector<int> v(10);
    std::iota(v.begin(), v.end(), 0);

    assert(boost::unique(v, greater_by_two()).size() == 5);
}