Assertion failure in rrb::Size::pop

[krobelus]

This program causes an assertion failure

fn main() {
    let mut x = im::Vector::new();
    for _ in 0..44 { x.push_back(0); }
    for _ in 0..20 { x.insert(0, 0); }
    x.insert(1, 0);
    for _ in 0..441 { x.push_back(0); }
    for _ in 0..58 { x.insert(0, 0); }
    x.insert(514, 0);
    for _ in 0..73 { x.push_back(0); }
    for _ in 0..10 { x.insert(0, 10); }
    x.insert(514, 0);
}

Here's what the tree looks like before the last insertion. It seems that the split is performed at the wrong chunk.
When we do x.insert(514, 0), we need to split at index 514 which corresponds to 514 - 4 - 64 = 446 within middle. Hence the split should be done in the last but one node, but it seems like the one before is used.

Should the middle be using a size table here?

length       648
middle_level 1
outer_f      [length = 4]
inner_f      [length = 64]
middle:
Nodes [length = 9], Size(507)
    Values [length = 2]
    Values [length = 64]
    Values [length = 64]
    Values [length = 64]
    Values [length = 64]
    Values [length = 64]
    Values [length = 64]
    Values [length = 64]
    Values [length = 57]
inner_b      [length = 64]
outer_b      [length = 9]