Divergent Equation
The double-X cost is the quiet engineering trick here: because both X's draw from the same value, buying back three cards costs seven mana and four costs nine, not the four or five a single would suggest. Regrowth effects for instants and sorceries have always been priced to keep them from becoming a resource loop, and this one solves the problem by making bulk retrieval scale steeply along the mana curve rather than capping the count outright. Return one card and you have paid three mana for a slightly awkward single-target recursion; return four and you have committed most of a late-game turn to refilling your hand. That curve is the whole design: it wants to sit idle in the early game and cash out once the graveyard is deep and the mana is there. The self-exile clause is what closes the loop it would otherwise open, since a card that rebuys instants and sorceries could otherwise pair with a second copy or another recursion spell to grind a graveyard indefinitely; exiling itself on resolution takes it out of the pool it feeds. What is left is a piece of blue card advantage tuned so its ceiling arrives only when you have earned the mana to reach it, patient by construction rather than by restriction.
