bt_base=bt_power
bt_power=bt_mine,bt_refinery,bt_barracks,bt_wall
bt_refinery=bt_barracks,bt_wall,bt_radar
bt_barracks=bt_tank,bt_gun,bt_hospital,bt_radar
bt_tank=bt_randd,bt_repair,bt_robot,bt_bmine,bt_supergun
bt_robot=bt_naval,bt_air
bt_randd=bt_power2,bt_advcomms,bt_chem
bt_advcomms=bt_silo,bt_shield
bt_air=bt_pad




