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/// A collection with a length.
pub trait Len {
    /// Returns the length of the collection.
    fn len(&self) -> usize;

    /// Returns whether the collection is empty.
    fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

/// A mutable collection which can be cleared.
pub trait LenZero: Len {
    /// Empties the collection, making its length zero.
    fn clear(&mut self);
}

/// A mutable collection with a length.
pub trait LenMut: LenZero {
    /// Truncates the collection so that it is no greater than `len` long.
    fn truncate(&mut self, len: usize);
}