def operation_at(self, qubit: cirq.Qid, moment_index: int) -> cirq.Operation | None:
        """Finds the operation on a qubit within a moment, if any.

        Args:
            qubit: The qubit to check for an operation on.
            moment_index: The index of the moment to check for an operation
                within. Allowed to be beyond the end of the circuit.

        Returns:
            None if there is no operation on the qubit at the given moment, or
            else the operation.
        """
        if not 0 <= moment_index < len(self.moments):
            return None
        return self.moments[moment_index].operation_at(qubit)
Finds the operation on a qubit within a moment, if any.

        Args:
            qubit: The qubit to check for an operation on.
            moment_index: The index of the moment to check for an operation
                within. Allowed to be beyond the end of the circuit.

        Returns:
            None if there is no operation on the qubit at the given moment, or
            else the operation.
        
cirq-core/cirq/circuits/circuit.py