impl CheckPoint {
    /// Returns the checkpoint at `height` if one exists, otherwise the nearest checkpoint at a
    /// lower height.
    ///
    /// This is equivalent to taking the "floor" of `height` over this checkpoint chain.
    ///
    /// Returns `None` if no checkpoint exists at or below the given height.
    pub fn floor_at(&self, height: u32) -> Option<Self> {
        self.range(..=height).next()
    }

    /// Returns the checkpoint located a number of heights below this one.
    ///
    /// This is a convenience wrapper for [`CheckPoint::floor_at`], subtracting `to_subtract` from
    /// the current height.
    ///
    /// - If a checkpoint exists exactly `offset` heights below, it is returned.
    /// - Otherwise, the nearest checkpoint *below that target height* is returned.
    ///
    /// Returns `None` if `to_subtract` is greater than the current height, or if there is no
    /// checkpoint at or below the target height.
    pub fn floor_below(&self, offset: u32) -> Option<Self> {
        self.floor_at(self.height().checked_sub(offset)?)
    }
}