Rule 615.7

Some prevention effects generated by the resolution of a spell or ability refer to a specific amount of damage—for example, “Prevent the next 3 damage that would be dealt to target creature or player this turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” creature or player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be dealt to the shielded creature or player by two or more applicable sources at the same time, the player or the controller of the creature chooses which damage the shield prevents. Once the shield has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount of damage; the number of events or sources dealing it doesn’t matter.

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