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pub trait Foldable: Sized {
    type Elm;

    fn fold_left<B, F>(&self, b: B, f: F) -> B
    where
        F: Fn(B, &Self::Elm) -> B;

    fn fold_right<B, F>(&self, b: B, f: F) -> B
    where
        F: Fn(&Self::Elm, B) -> B;
}

impl<A> Foldable for Vec<A> {
    type Elm = A;

    fn fold_left<B, F>(&self, b: B, f: F) -> B
    where
        F: Fn(B, &Self::Elm) -> B,
    {
        self.iter().fold(b, f)
    }

    fn fold_right<B, F>(&self, b: B, f: F) -> B
    where
        F: Fn(&Self::Elm, B) -> B,
    {
        self.iter().rev().fold(b, |x, y| f(y, x))
    }
}