Cthulhu code

std::copy_if(a.begin(), a.end(), std::back_inserter(c),
  std::bind(std::less<>(),
    std::bind(
      std::count<std::vector<int>::iterator, int>,
        std::bind(
          static_cast<std::vector<int>::iterator (std::vector<int>::*)()>(&std::vector<int>::begin),
          &c
        ),
        std::bind(
          static_cast<std::vector<int>::iterator (std::vector<int>::*)()>(&std::vector<int>::end),
          &c
        ),
        std::placeholders::_1
    ),
    std::bind(
      std::minus<>(),
        std::bind(
          std::count<std::vector<int>::iterator, int>,
            a.begin(),
            a.end(),
            std::placeholders::_1
        ),
        std::bind(
          std::count<std::vector<int>::iterator, int>,
            b.begin(),
            b.end(),
            std::placeholders::_1
        )
    )
  )
);