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();
        }
    }
Stocks up the right child, assuming the left child isn't underfull, and
provisions an extra element to all
benchmarks/haystacks/code/rust-library.rs