code: def _one_q_pauli_prep(label, index, qubit):
      
      if index not in [0, 1]:
          raise ValueError(f'Bad Pauli index: {index}')
  
      if label == 'X':
          if index == 0:
              return Program(RY(pi / 2, qubit))
          else:
              return Program(RY(-pi / 2, qubit))
  
      elif label == 'Y':
          if index == 0:
              return Program(RX(-pi / 2, qubit))
          else:
              return Program(RX(pi / 2, qubit))
  
      elif label == 'Z':
          if index == 0:
              return Program()
          else:
              return Program(RX(pi, qubit))
  
      raise ValueError(f'Bad Pauli label: {label}')
docs: Prepare the index-th eigenstate of the pauli operator given by label.