At What kind of topological invariants can you get from just Hopf algebras?, link [4] is not being processed:

However, it is there in the source:

```
[1]: http://www-math.mit.edu/~ptingley/publications/minus-sign.pdf
[2]: https://arxiv.org/abs/math/0503040
[3]: http://math.columbia.edu/~ellis/ugrad_seminar_spring_2011/geissinger.pdf
[4]: http://garsia.math.yorku.ca/~zabrocki/talks/Hopfzoo.pdf
```

Why isn't it processed?