code: def get_bitstring_from_index(index, qubit_num):
      
      if index > (2**qubit_num - 1):
          raise IndexError("Index {} too large for {} qubits.".format(index, qubit_num))
      return bin(index)[2:].rjust(qubit_num, '0')
docs: Returns the bitstring in lexical order that corresponds to the given index in 0 to 2^(qubit_num)
      :param int index:
      :param int qubit_num:
      :return: the bitstring
      :rtype: str