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You have learned a trick that makes your spellcasting more flexible.
Prerequisites: Spell Mastery, wizard level 1st.
Benefit: When you prepare your spells each day, you can choose to keep a number of spell slots equal to your Intelligence modifier (minimum 1) flexibly prepared. Instead of preparing a single spell within each of these slots, you can partially prepare two spells. Anytime after you prepare these flexible slots, you can spend a full-round action to finalize one slot, choosing one of the two spells to be finished and fully prepared in that slot.