Rule 608.2f

If an effect gives a player the option to pay mana, he or she may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, he or she does so by putting that spell on top of the stack, then continuing to cast it by following the steps in rules 601.2a–h, except no player receives priority after it’s cast. The currently resolving spell or ability then continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.

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