When I was planning the project, I looked into FactGraph to see if it'd be worth building on. I opted not to for a few reasons:

- It's oriented toward the IRS's internal XML API in a way that's not really useful for this project, since no consumer of my code is going to digitally submit to the IRS.

- The semantics of FactGraph are largely undefined, and the maintainers themselves seem to acknowledge that its development trajectory went off in a bad direction and it needs major divisions.

- It's super verbose.

- It's the year of Lean, so I wanted to find an excuse to use Lean.

Other work has been done in this domain, though. The biggest example is Catala[0], though Prof. Sarah Lawsky at Northwestern seems to have made some strides toward formalizing the Internal Revenue Code.

[0] https://catala-lang.org/