diff --git a/README.md b/README.md
index d60edea..435439e 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,14 @@ Every example is checked against wanderbooks on every `cargo test`
 with fails the build, so the examples can't rot. An expected value is worked
 by hand from the cited source, never copied from an engine's output.
 
+The schema, the parser (`Example::from_toml`, every structure with
+`deny_unknown_fields`) and the `Engine` trait an accounting engine
+implements live in `bookspec/checker`; `bookspec/runner` runs any engine
+through the examples and reports per rule: pass, fail or not claimed. The
+wanderbooks adapter is `crates/ledger-core/tests/bookspec.rs`. The rule
+identifiers are `BS-<area>-<n>`, tabled in `bookspec/spec/rules-index.md`
+(P13 steps 1 to 3).
+
 ## The shape of an example
 
 One TOML file per example in `examples/`, named for its `id`.
@@ -99,7 +107,7 @@ declares `largest-remainder`, the rule a members close splits by.
 authorities alone (a standard, a regulation, a statute, a court, the
 mechanics of double entry); `CONVENTION`, common practice where no
 authority governs (rules.md's CONVENTION); `CHOICE`, an owner's or the
-house's decision (a plan D-number, or a DECISION rules.md records) that
+house's decision (a plan D-number, or a CHOICE rules.md records) that
 some expected value depends on, even when authorities carry the rest. An
 example whose figures follow a decision cites it in `sources`.
 
diff --git a/checker/tests/docs.rs b/checker/tests/docs.rs
new file mode 100644
index 0000000..d27d8de
--- /dev/null
+++ b/checker/tests/docs.rs
@@ -0,0 +1,28 @@
+//! The documentation bookspec's identifiers live in: `docs/spec/rules.md`
+//! carries each stated rule's id and speaks bookspec's `CHOICE`.
+
+use std::path::Path;
+
+use bookspec_checker::rules;
+
+/// `docs/spec/rules.md` carries every stated rule's id under its heading,
+/// and no longer calls a choice a decision (P13 step 3).
+#[test]
+fn rules_md_carries_every_id() {
+    let text = std::fs::read_to_string(
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("../../docs/spec/rules.md"),
+    )
+    .expect("docs/spec/rules.md");
+    for rule in rules().iter().filter(|r| r.status.is_some()) {
+        assert!(
+            text.contains(&format!("**bookspec:** `{}`", rule.id)),
+            "B-{} carries {}",
+            rule.b,
+            rule.id
+        );
+    }
+    assert!(
+        !text.contains("DECISION"),
+        "rules.md speaks bookspec's CHOICE"
+    );
+}