code: def cbpdn_ystep(k):
      
  
      AXU = mp_Z_X[k] + mp_Z_U[k]
      mp_Z_Y[k] = sp.prox_l1(AXU, (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.