docs: def rmse_2pi(x, xhat):
      ''' Calcualte rmse between vector or matrix x and xhat, ignoring mod of 2pi.'''
      real_diff = from_0_to_pi(x - xhat)
      np.square(real_diff, out=real_diff)
      sum_ = np.sum(real_diff)
      return sqrt(sum_ / len(x))