template <typename T>
T stdev(std::vector<T> const& v) {
  T mean = v[0];
  T variance = 0;
  for (size_t k = 1; k < v.size(); ++k) {
    T meanPre = mean;
    mean     += (v[k] - mean) / k;
    variance += (v[k] - mean) * (v[k] - meanPre);
  }
  return std::sqrt(variance/v.size());
}