You should use your brain and think

QUOTE
TRIGGER SPAWNS:
Bring(CurrentPlayer, AtLeast, 1, Civilian, Archon);
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RemoveUnitAtLocation(1, Civilian, CurrentPlayer, Archon);
CreateUnit(1, Archon, Unit Area, CurrentPlayer);
CreateUnit(1, Siege Tank Tank Mode, Red 2, CurrentPlayer);
PreserveTrigger();
Comment("Archon Spawn @ Unit Area");
Bring(CurrentPlayer, AtLeast, 1, Civilian, Firebat);
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RemoveUnitAtLocation(1, Civilian, CurrentPlayer, Firebat);
CreateUnit(1, Firebat, Unit Area, CurrentPlayer);
CreateUnit(1, Siege Tank Tank Mode, Red 2, CurrentPlayer);
PreserveTrigger();
Comment("Firebat Spawn @ Unit Area");
Bring(CurrentPlayer, AtLeast, 1, Civilian, Vulture);
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RemoveUnitAtLocation(1, Civilian, CurrentPlayer, Vulture);
CreateUnit(1, Vulture, Unit Area, CurrentPlayer);
CreateUnit(1, Siege Tank Tank Mode, Red 2, CurrentPlayer);
PreserveTrigger();
Comment("Vulture Spawn @ Unit Area");
TRIGGER TWO:
Command(CurrentPlayer, AtLeast, 1, Siege Tank Siege Mode);
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MoveLocation(TankLocation, Siege Tank Siege Mode, CurrentPlayer, Anywhere);
RemoveUnitAtLocation(1, Siege Tank Siege Mode, CurrentPlayer, TankLocation);
MoveUnit(1, AnyUnit, CurrentPlayer, Unit Area, TankLocation);
PreserveTrigger();
Comment("Any Unit from Unit Area");
You need to make sure they have a siege tank in siege mode before moving the location on to it.