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.
In reply to@mightyiam:matrix.org 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
How does a user specify input? What kind of output does does a doc tester produce? Would you find it useful to co-opt the language/structure in the issue linked above, or does it pose unnecessary constraints on your work?