Let's say you want the Marine to be allowed to be built at X seconds into the game.
Give the player a Barracks and have him have enough minerals to build it.
Let's say you want the player to be allowed to build a Firebat but not a Medic.
Give the player a Barracks and an Academy and have him enough minerals to build a firebat and not enough to build a medic. Or make a trigger that removes all medics and refunds him.
Let's say you want to allow the player to build a Barracks.
Give the player a Command Center.
It's not that complicated
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