Merge pull request #81 from fiddlerwoaroof/wave/core-formula-io

core/formula/io: a duplicate-section round trip, the WHERE word boundary, and a parameter that promised a filter

context/test-evidence/core.md

@@ -12,6 +12,20 @@ none.
 
 ## `model/types.rs`
 
+### a_hidden_dimension_list_reduces_to_one_of_two_states
+
+improvise-5jm5. `evaluate_aggregated` took `none_cats: &[String]` and consulted
+it once, for emptiness — so two lists with different contents and the same
+emptiness gave the same answer, while the name promised a filter. Nothing
+misbehaved; the signature described something the body never did.
+
+This test is the *smaller* half of the guard and says so. The real guard is the
+signature: the defect the ticket describes is no longer constructible, exactly
+as `Checked`'s constructor is the guard for the basis check rather than a test
+that can fail. What this test does add is that `Aggregation::of` is the only
+place the list is read, and that a list of two hidden dimensions is not somehow
+a third state — the reduction is emptiness, and the enum is not a count.
+
 ### apply_agg_over_nothing_is_absent_for_every_function
 
 An empty cell must read as *absent*, not as zero — otherwise every blank