def qubit_index_to_equivalence_group_key(self, index: int) -> int:
        """Returns a key that differs between non-interchangeable qubits."""
        if (_zero_mod_pi(self.zeta) or self._zeta_insensitive()) and (
            _zero_mod_pi(self.chi) or self._chi_insensitive()
        ):
            return 0
        return index
Returns a key that differs between non-interchangeable qubits.
cirq-core/cirq/ops/fsim_gate.py