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At the beginning of your upkeep, you may exchange control of target nonland permanent you control and target nonland permanent an opponent controls with equal or lesser mana valuemana valueThe total amount of mana in a card's cost, ignoring colour. A card costing 
has mana value 4. X counts as 0 except on the stack..
Hover an underlined rule to see what it means.
Read from the rules text. A card that enables a keyword makes or is the thing; a card that rewards it gets better when you have it. Two cards interact when one enables what the other rewards.
