What is the system of choosing games in Proleague?

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In a standard match of Starcraft Proleague, two teams compete in four different games, with players and maps chosen beforehand, and play out an additional "Ace Match" in case of the tie, with players chosen on the spot.
How are the players and maps for the first four games chosen..? There is definitely a lot of strategy to it, so what are the rules governing it..?
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