void time_test(const S1& s1, const S2& s2, S3& s3, Op op, size_t n, size_t m) {
typedef typename S1::iterator I;
timer t;
t.start();
for (int i = 0; i < m; ++i)
std::transform(s1.begin(), s1.end(), s2.begin(), s3.begin(), op);
double time = t.stop()/(double(n) * m);
std::cout << std::setw(6) << std::fixed << std::setprecision(2) << time << "\t";
}
static inline void set_difference(const S1 &src1, const S2 &src2, D &dst) {
std::set_difference(
src1.begin(), src1.end(), src2.begin(), src2.end(),
std::inserter(dst, dst.begin()));
}