code: def design_matrix(btens):
      """Calculate the design matrix from the b-tensors.
  
      Parameters
      ----------
      btens : numpy.ndarray
          An array of b-tensors of shape (number of acquisitions, 3, 3).
  
      Returns
      -------
      X : numpy.ndarray
          Design matrix.
  
      Notes
      -----
      The design matrix is generated according to
  
          .. math::
  
              X = \begin{pmatrix} 1 & -\mathbf{b}_1^T & \frac{1}{2}(\mathbf{b}_1
              \otimes\mathbf{b}_1)^T \\ \vdots & \vdots & \vdots \\ 1 &
              -\mathbf{b}_n^T & \frac{1}{2}(\mathbf{b}_n\otimes\mathbf{b}_n)^T
              \end{pmatrix}
      """
      X = np.zeros((btens.shape[0], 28))
      for i, bten in enumerate(btens):
          b = from_3x3_to_6x1(bten)
          b_sq = from_6x6_to_21x1(b @ b.T)
          X[i] = np.concatenate(
              ([1], (-b.T)[0, :], (0.5 * b_sq.T)[0, :])
          )
      return X
  
imports: from warnings import warn
  import numpy
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import auto_attr
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 9 for calling a global function with example usage: design_matrix(btens) and returns: X