code: def cnst_AT(self, Y):
          r
  
          return self.cnst_A0T(self.block_sep0(Y)) + \
              self.cnst_A1T(self.block_sep1(Y))
docs: r"""Compute :math:`A^T \mathbf{y}` where
  
          .. math::
             A^T \mathbf{y} = \left( \begin{array}{cc} A_0^T & A_1^T
             \end{array} \right) \left( \begin{array}{c} \mathbf{y}_0
             \\ \mathbf{y}_1 \end{array} \right) = A_0^T \mathbf{y}_0 +
             A_1^T \mathbf{y}_1 \;\;.