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treesOfSize : ℕ → List(BT)
treesOfSize(0) = [left(■)]
treesOfSize(k + 1) = [right(l, r) | x in [0 .. k], l in treesOfSize(x), r in treesOfSize(k .- x)]
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treeFold : r11 × (ℕ × r11 × r11 → r11) × Tree → r11
treeFold(x, f, left(■)) = x
treeFold(x, f, right(n, l, r)) = f(n, treeFold(x, f, l), treeFold(x, f, r))