code: def par_y0bstep(i):
      r
  
      global mp_Y0
      mp_Y0[i] = 1/mp_rho*mp_S + mp_DX[i] + mp_U0[i] + mp_b
docs: r"""The parallel component of the step to minimise the augmented
      Lagrangian with respect to :math:`\mathbf{y}_0`.
  
      Parameters
      ----------
      i : int
        Index of grouping to update