code: def c2r(self):
          
          return matrix(a=self.tt.__complex_op('M'), n=_np.concatenate(
              (self.n, [2])), m=_np.concatenate((self.m, [2])))
docs: Get real matrix from complex one suitable for solving complex linear system with real solver.
  
          For matrix :math:`M(i_1,j_1,\\ldots,i_d,j_d) = \\Re M + i\\Im M` returns (d+1)-dimensional matrix
          :math:`\\tilde{M}(i_1,j_1,\\ldots,i_d,j_d,i_{d+1},j_{d+1})` of form
          :math:`\\begin{bmatrix}\\Re M & -\\Im M \\\\ \\Im M &  \\Re M  \\end{bmatrix}`. This function
          is useful for solving complex linear system :math:`\\mathcal{A}X = B` with real solver by
          transforming it into
  
          .. math::
             \\begin{bmatrix}\\Re\\mathcal{A} & -\\Im\\mathcal{A} \\\\
                             \\Im\\mathcal{A} &  \\Re\\mathcal{A}  \\end{bmatrix}
             \\begin{bmatrix}\\Re X \\\\ \\Im X\\end{bmatrix} =
             \\begin{bmatrix}\\Re B \\\\ \\Im B\\end{bmatrix}.