docs: def _validate_num_qubits(state: np.ndarray) -> int:
      """Validates that state's size is a power of 2, returning number of qubits.
      """
      size = state.size
      if size & (size - 1):
          raise ValueError('state.size ({}) is not a power of two.'.format(size))
      return size.bit_length() - 1