docs: def euclidean(x, y):
      """Standard euclidean distance.
  
      ..math::
          D(x, y) = \sqrt{\sum_i (x_i - y_i)^2}
      """
      result = 0.0
      for i in range(x.shape[0]):
          result += (x[i] - y[i]) ** 2
      return np.sqrt(result)