code: def operation_at(self,
                       qubit: ops.Qid,
                       moment_index: int) -> Optional[ops.Operation]:
          
          if not 0 <= moment_index < len(self._moments):
              return None
          for op in self._moments[moment_index].operations:
              if qubit in op.qubits:
                  return op
          return None
docs: 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.