Xboom_bug, /* bug explosion; transition to Zbug */
Xboom_tank, /* tank/alien/bomb explosion; transition to Ztank */
Xboom_android, /* android explosion; transition to Xboom_2 */
Xboom_bug, /* bug explosion; transition to Zbug */
Xboom_tank, /* tank/alien/bomb explosion; transition to Ztank */
Xboom_android, /* android explosion; transition to Xboom_2 */