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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]