docs: def _unavailable_range(self):
          """
          Return a generator for the indices of the unavailable region of
          ``_reference_queue``.
          """
          with self._lock:
              i = self._resource_end
              j = self._resource_start
              if j < i or self.empty():
                  j += self.maxsize
  
              for k in range(i, j):
                  yield k % self.maxsize