def cbpdnmd_ustep(k):
    """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.
    """

    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]
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.