docs: def _iterate_to_update_x_transforms(self):
          """Perform the inner iteration."""
          self._inner_iters = 0
          self._last_inner_error = float('inf')
          while self._inner_error_is_decreasing():
              print('  Starting inner iteration {0:03d}. Current err = {1:12.5E}'
                    ''.format(self._inner_iters, self._last_inner_error))
              self._update_x_transforms()
              self._inner_iters += 1