/* Use unique_ptr because there's only one root per stage so it's pointless
to put it in a pool. */
std::unique_ptr<Root> root;
std::vector<std::unique_ptr<System>> systems;
/* Use unique_ptr because there's only one root per stage so it's pointless
to put it in a pool. */
std::unique_ptr<Root> root;
std::vector<std::unique_ptr<System>> systems;