code: def qubit_pairs_to_qubit_order(
      qubit_pairs: Sequence[Sequence[ops.Qid]]
      ) -> List[ops.Qid]:
      
  
      if set(len(qubit_pair) for qubit_pair in qubit_pairs) != set((2,)):
          raise ValueError(
              'set(len(qubit_pair) for qubit_pair in qubit_pairs) != '
              'set((2,))')
      n_pairs = len(qubit_pairs)
      qubits = [] # type: List[ops.Qid]
      for i in range(0, 2 * (n_pairs // 2), 2):
          qubits += [qubit_pairs[i][0], qubit_pairs[i + 1][0],
                     qubit_pairs[i][1], qubit_pairs[i + 1][1]]
      if n_pairs % 2:
          qubits += list(qubit_pairs[-1])
      return qubits
docs: Takes a sequence of qubit pairs and returns a sequence in which every
      pair is at distance two.
  
      Specifically, given pairs (1a, 1b), (2a, 2b), etc. returns
      (1a, 2a, 1b, 2b, 3a, 4a, 3b, 4b, ...).