def from_mixture(
        mixture: protocols.SupportsMixture, key: str | cirq.MeasurementKey | None = None
    ):
        """Creates a copy of a mixture with the given measurement key."""
        return MixedUnitaryChannel(mixture=list(protocols.mixture(mixture)), key=key)
Creates a copy of a mixture with the given measurement key.
cirq-core/cirq/ops/mixed_unitary_channel.py