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    mod append;
    mod borrow;
    pub mod map;
    mod mem;
    mod merge_iter;
    mod navigate;
    mod node;
    mod remove;
    mod search;
    pub mod set;
    mod split;

    #[doc(hidden)]
    trait Recover<Q: ?Sized> {
        type Key;

        fn get(&self, key: &Q) -> Option<&Self::Key>;
        fn take(&mut self, key: &Q) -> Option<Self::Key>;
        fn replace(&mut self, key: Self::Key) -> Option<Self::Key>;
    }

    /// Same purpose as `Option::unwrap` but doesn't always guarantee a panic
    /// if the option contains no value.
    /// SAFETY: the caller must ensure that the option contains a value.
    #[inline(always)]
    pub unsafe fn unwrap_unchecked<T>(val: Option<T>) -> T {
        val.unwrap_or_else(|| {
            if cfg!(debug_assertions) {
                panic!("'unchecked' unwrap on None in BTreeMap");
            } else {
                unsafe {
                    core::intrinsics::unreachable();
                }
            }
        })
    }

    #[cfg(test)]
    /// XorShiftRng
    struct DeterministicRng {
        count: usize,
        x: u32,
        y: u32,
        z: u32,
        w: u32,
    }

    #[cfg(test)]
    impl DeterministicRng {
        fn new() -> Self {
            DeterministicRng { count: 0, x: 0x193a6754, y: 0xa8a7d469, z: 0x97830e05, w: 0x113ba7bb }
        }

        /// Guarantees that each returned number is unique.
        fn next(&mut self) -> u32 {
            self.count += 1;
            assert!(self.count <= 70029);
            let x = self.x;
            let t = x ^ (x << 11);
            self.x = self.y;
            self.y = self.z;
            self.z = self.w;
            let w_ = self.w;
            self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
            self.w
        }
    }