docs: def error_perturbation_var(C, S):
      r"""Error-perturbation arising from a given sensitivity
  
      Parameters
      ----------
      C : (M, M) ndarray
          Count matrix
      S : (K, M, M) ndarray
          Sensitivity tensor
  
      """
      K = S.shape[0]
      cov = tmatrix_cov(C)
      for i in range(K):
          R = S[i, :, :]
          X[i] = (R[:, :, np.newaxis] * cov * R[:, np.newaxis, :]).sum()