Add Todd Arbagast's elements
https://www.oden.utexas.edu/media/reports/2017/1728.pdf
Also (more recent work): http://users.oden.utexas.edu/~arbogast/PaperArchive/Arbogast_Tao_Wang_2021X.pdf
Also #59
I think #92 did this?
I'm currently unsure about how correct the elements in #92 are, so leaving this open until I've reread both links
Ah, I see.
See also https://arxiv.org/pdf/1807.01416.pdf (Hexahedral div elements) and https://www.doi.org/10.1137/15M1013705 (AC div elements) and https://arxiv.org/pdf/1809.02192.pdf (direct serendipity)