TOID(struct my_root) root = POBJ_ROOT(pop);
TX_BEGIN(pop) {
  TX_ADD(root);
  TOID(struct rectangle) rect = TX_NEW(struct rectangle);
  D_RW(rect)->x = 5;
  D_RW(rect)->y = 10;
  D_RW(root)->rect = rect;
} TX_END

int p = area_calc(D_RO(root)->rect);