code: def cbpdnmd_ustep(k):
      
  
      mp_Z_U0[k] += mp_DX[k] - mp_Z_Y0[k] - mp_S[k]
      mp_Z_U1[k] += mp_Z_X[k] - mp_Z_Y1[k]
docs: Do the U 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.