void Generation::compact() {

  {- -------------------------------------------
  (1) Generation 中の全ての CompactibleSpace に対して CompactibleSpace::compact() を呼び出すだけ.
      ---------------------------------------- -}

      CompactibleSpace* sp = first_compaction_space();
      while (sp != NULL) {
        sp->compact();
        sp = sp->next_compaction_space();
      }
    }