1# Chapter 6½: inside rules/src
2
3Two files. `lib.rs` is the whole engine and every rule, in about 500 lines,
4and it reads well from top to bottom:
5
61. **The vocabulary.** `Kind` (yes-or-no, pick, how-much, how-likely),
7   `Fact` (everything that can be known about a question), `Value`, and
8   `Source` (who teaches a fact: Jev, the LLM, or Jev again).
92. **What a rule can say.** `Test` (a fact is a value, or is known), and
10   `Then`: an `End` (stop with this), an `Effect` (go and make a call), or a
11   `Note` (show this; list it).
123. **`RULES`.** The rules themselves, as data. This is the list in
13   chapter 6, and the one place to change what the site does.
144. **`Known`.** What has been learned so far about one question.
155. **`Network`.** `compile` builds the Rete network from the rules;
16   `next` and `decide` say what to do; `alpha`, `join` and `used` are what
17   the diagram draws.
18
19`tests.rs` holds the engine to its promises, including the one that matters
20most: every way the Jev facts can fall ends with something to show or
21something to do (`every_way_the_jev_facts_can_fall_ends_or_does_something`).
22
23> **Try it.** `cargo test -p rules` runs them, offline, in well under a
24> second.
25
26| File | What |
27| --- | --- |
28| [lib.rs](lib.rs) | Facts, tests, rules, `RULES`, `Known`, and the `Network` that compiles and runs them. |
29| [tests.rs](tests.rs) | The engine's tests: each kind of question takes its path, rules share the tests they have in common, a failed test fails what follows it, and every answerable outcome is covered. |
30
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