void PSYoungGen::swap_spaces() {

  {- -------------------------------------------
  (1) _from_space と _to_space の値を入れ替える.
      ---------------------------------------- -}

      MutableSpace* s    = from_space();
      _from_space        = to_space();
      _to_space          = s;

  {- -------------------------------------------
  (1) ついでに, _from_mark_sweep と _to_mark_sweep の値も入れ替える.
      ---------------------------------------- -}

      // Now update the decorators.
      PSMarkSweepDecorator* md = from_mark_sweep();
      _from_mark_sweep           = to_mark_sweep();
      _to_mark_sweep             = md;

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert(from_mark_sweep()->space() == from_space(), "Sanity");
      assert(to_mark_sweep()->space() == to_space(), "Sanity");
    }