Prunes the cataloged observations.

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