413.2e.

If an effect gives a player the option to pay mana, he or she may play mana abilities before taking that action. If an effect specifically instructs or allows a player to play a spell during resolution, he or she does so by putting that spell on top of the stack, then continuing to play it by following the steps in rules 409.1a-i (except no player receives priority after it's played). The currently resolving spell or ability then continues to resolve, which may include playing other spells this way. No other spells or abilities can normally be played during resolution.