code: def cbpdnmd_ystep(k):
      
  
      if mp_W.shape[0] > 1:
          W = mp_W[k]
      else:
          W = mp_W
      AXU0 = mp_DX[k] - mp_S[k] + mp_Z_U0[k]
      AXU1 = mp_Z_X[k] + mp_Z_U1[k]
      mp_Z_Y0[k] = mp_xrho*AXU0 / (W**2 + mp_xrho)
      mp_Z_Y1[k] = sp.prox_l1(AXU1, (mp_lmbda/mp_xrho))
docs: Do the Y step of the cbpdn stage. The only parameter is the slice
      index `k` and there are no return values; all inputs and outputs are
      from and to global variables.