code: def y0astep():
      r
  
      global mp_b
      mp_b[:] = mp_inv_off_diag * np.sum((mp_S + mp_rho*(mp_DX+mp_U0)),
                                         axis=mp_axisM, keepdims=True)
docs: r"""The serial component of the step to minimise the augmented
      Lagrangian with respect to :math:`\mathbf{y}_0`.