Sorry, there's no documentation. There are tests for the repl examples and there are tests for the WIP expression examples.
https://github.com/mobusoperandi/eelco/tree/0eb9042220befe188aa4399ffc6615579cebdd5f/crates/eelco/tests
If it evaluates into null then it's good. If it evaluates into anything else it's bad. If it fails to evaluate, it's bad. The idea is that the author will use Nix assertions to prove their points in the example.