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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 evolve(self, rho: Density) -> Density: """Apply the action of this channel upon a density"""

        N = rho.qubit_nb
        qubits = rho.qubits

        indices = list([qubits.index(q) for q in self.qubits]) + \
            list([qubits.index(q) + N for q in self.qubits])

        tensor = bk.tensormul(self.tensor, rho.tensor, indices)
        return Density(tensor, qubits, rho.memory)