`BookProof.ChapterFreeFieldBornSignOrientationCard.natCard_even_flip` (n : ℕ) : Nat.card {b : Fin (n + 1) → Bool // Even (flipCount b)} = 2 ^ n

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`BookProof.ChapterFreeFieldBornSignOrientationCard.natCard_even_flip` (n : ℕ) : Nat.card {b : Fin (n + 1) → Bool // Even (flipCount b)} = 2 ^ n · Prove2Me