A Forgetful Brain

You are the only judge at a Competitive Legacy event. Anton and Nicolas are in the middle of a tense game and spells are flying all over the place. At the end of Nicolas’s turn, Anton plays a Brainstorm. Nicolas says fine. Anton then draws three cards and, without returning any cards to the top of the library, he untaps and draws for his turn. Nicolas stops him after the card is already drawn and calls you over. You determine this was an honest mistake.

What is your ruling?

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Answer coming soon!