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    use super::map::MIN_LEN;
    use super::node::{marker, ForceResult::*, Handle, LeftOrRight::*, NodeRef, Root};

    impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
        /// Stocks up a possibly underfull node by merging with or stealing from a
        /// sibling. If succesful but at the cost of shrinking the parent node,
        /// returns that shrunk parent node. Returns an `Err` if the node is
        /// an empty root.
        fn fix_node_through_parent(
            self,
        ) -> Result<Option<NodeRef<marker::Mut<'a>, K, V, marker::Internal>>, Self> {
            let len = self.len();
            if len >= MIN_LEN {
                Ok(None)
            } else {
                match self.choose_parent_kv() {
                    Ok(Left(mut left_parent_kv)) => {
                        if left_parent_kv.can_merge() {
                            let parent = left_parent_kv.merge_tracking_parent();
                            Ok(Some(parent))
                        } else {
                            left_parent_kv.bulk_steal_left(MIN_LEN - len);
                            Ok(None)
                        }
                    }
                    Ok(Right(mut right_parent_kv)) => {
                        if right_parent_kv.can_merge() {
                            let parent = right_parent_kv.merge_tracking_parent();
                            Ok(Some(parent))
                        } else {
                            right_parent_kv.bulk_steal_right(MIN_LEN - len);
                            Ok(None)
                        }
                    }
                    Err(root) => {
                        if len > 0 {
                            Ok(None)
                        } else {
                            Err(root)
                        }
                    }
                }
            }
        }
    }

    impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
        /// Stocks up a possibly underfull node, and if that causes its parent node
        /// to shrink, stocks up the parent, recursively.
        /// Returns `true` if it fixed the tree, `false` if it couldn't because the
        /// root node became empty.
        ///
        /// This method does not expect ancestors to already be underfull upon entry
        /// and panics if it encounters an empty ancestor.
        pub fn fix_node_and_affected_ancestors(mut self) -> bool {
            loop {
                match self.fix_node_through_parent() {
                    Ok(Some(parent)) => self = parent.forget_type(),
                    Ok(None) => return true,
                    Err(_) => return false,
                }
            }
        }
    }

    impl<K, V> Root<K, V> {
        /// Removes empty levels on the top, but keeps an empty leaf if the entire tree is empty.
        pub fn fix_top(&mut self) {
            while self.height() > 0 && self.len() == 0 {
                self.pop_internal_level();
            }
        }

        /// Stocks up or merge away any underfull nodes on the right border of the
        /// tree. The other nodes, those that are not the root nor a rightmost edge,
        /// must already have at least MIN_LEN elements.
        pub fn fix_right_border(&mut self) {
            self.fix_top();
            if self.len() > 0 {
                self.borrow_mut().last_kv().fix_right_border_of_right_edge();
                self.fix_top();
            }
        }

        /// The symmetric clone of `fix_right_border`.
        pub fn fix_left_border(&mut self) {
            self.fix_top();
            if self.len() > 0 {
                self.borrow_mut().first_kv().fix_left_border_of_left_edge();
                self.fix_top();
            }
        }

        /// Stock up any underfull nodes on the right border of the tree.
        /// The other nodes, those that are not the root nor a rightmost edge,
        /// must be prepared to have up to MIN_LEN elements stolen.
        pub fn fix_right_border_of_plentiful(&mut self) {
            let mut cur_node = self.borrow_mut();
            while let Internal(internal) = cur_node.force() {
                // Check if right-most child is underfull.
                let mut last_kv = internal.last_kv().consider_for_balancing();
                debug_assert!(last_kv.left_child_len() >= MIN_LEN * 2);
                let right_child_len = last_kv.right_child_len();
                if right_child_len < MIN_LEN {
                    // We need to steal.
                    last_kv.bulk_steal_left(MIN_LEN - right_child_len);
                }

                // Go further down.
                cur_node = last_kv.into_right_child();
            }
        }
    }

    impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV> {
        fn fix_left_border_of_left_edge(mut self) {
            while let Internal(internal_kv) = self.force() {
                self = internal_kv.fix_left_child().first_kv();
                debug_assert!(self.reborrow().into_node().len() > MIN_LEN);
            }
        }

        fn fix_right_border_of_right_edge(mut self) {
            while let Internal(internal_kv) = self.force() {
                self = internal_kv.fix_right_child().last_kv();
                debug_assert!(self.reborrow().into_node().len() > MIN_LEN);
            }
        }
    }

    impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::KV> {
        /// Stocks up the left child, assuming the right child isn't underfull, and
        /// provisions an extra element to allow merging its children in turn
        /// without becoming underfull.
        /// Returns the left child.
        fn fix_left_child(self) -> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
            let mut internal_kv = self.consider_for_balancing();
            let left_len = internal_kv.left_child_len();
            debug_assert!(internal_kv.right_child_len() >= MIN_LEN);
            if internal_kv.can_merge() {
                internal_kv.merge_tracking_child()
            } else {
                // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
                let count = (MIN_LEN + 1).saturating_sub(left_len);
                if count > 0 {
                    internal_kv.bulk_steal_right(count);
                }
                internal_kv.into_left_child()
            }
        }

        /// Stocks up the right child, assuming the left child isn't underfull, and
        /// provisions an extra element to allow merging its children in turn
        /// without becoming underfull.
        /// Returns wherever the right child ended up.
        fn fix_right_child(self) -> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
            let mut internal_kv = self.consider_for_balancing();
            let right_len = internal_kv.right_child_len();
            debug_assert!(internal_kv.left_child_len() >= MIN_LEN);
            if internal_kv.can_merge() {
                internal_kv.merge_tracking_child()
            } else {
                // `MIN_LEN + 1` to avoid readjust if merge happens on the next level.
                let count = (MIN_LEN + 1).saturating_sub(right_len);
                if count > 0 {
                    internal_kv.bulk_steal_left(count);
                }
                internal_kv.into_right_child()
            }
        }
    }