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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 choi(self) -> bk.BKTensor: """Return the Choi matrix representation of this super operator"""

        # Put superop axes in [ok, ib, ob, ik] and reshape to matrix
        N = self.qubit_nb
        return bk.reshape(self.sharp.tensor, [2**(N*2)] * 2)