docs: def _compress(self):
          """Prunes the cataloged observations."""
          rank = 0.0
          current = self._head
  
          while current and current._successor:
              if current._rank + current._successor._rank + current._successor._delta <= self._invariant(rank, self._observations):
                  removed = current._successor
  
                  current._value = removed._value
                  current._rank += removed._rank
                  current._delta = removed._delta
                  current._successor = removed._successor
  
              rank += current._rank
              current = current._successor