abstract syntax trees for expressions

https://tug.org/TUGboat/tb12-3-4/tb33arnon.pdf

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.32.5712&rep=rep1&type=pdf

https://pdfs.semanticscholar.org/7214/d4805660042521d4b825eb3324742b215072.pdf

http://mathlex.org/doc/how-mathlex-works

https://calculem.us/abstract-binding-trees-1/
https://semantic-domain.blogspot.com/2015/03/abstract-binding-trees.html
https://arxiv.org/abs/1601.06298