Panic through internalError on a disagreement key with no fix

datatype.go

@@ -198,7 +198,15 @@ func disagreement(a *template, da DataType, b *template, db DataType) error {
 		d.fix, d.other, d.want = b, a, da
 	}
 	proven := (&valueProof{}).proveColumnItem(d.fix).not[d.want] == ""
-	return disagreements[disagreementKey{kindOf(d.fix), kindOf(d.other), proven}](d)
+	return disagreementFix(disagreementKey{kindOf(d.fix), kindOf(d.other), proven})(d)
+}
+
+func disagreementFix(k disagreementKey) func(d itemPair) error {
+	fix, ok := disagreements[k]
+	if !ok {
+		panic(internalError("no disagreement names the fix for %+v", k))
+	}
+	return fix
 }
 
 func holdOne(d itemPair) error {

todo.md

@@ -26,8 +26,6 @@ least modify.
 
 This round, in order:
 
-- Test that `disagreements` holds every key `disagreement` can build, and panic through
-  `internalError` on a miss: a missing key is a nil func call today.
 - Name `printedNumber`'s `dp` cases, and say on `builtin` what `prints` and
   `proveNumber` a new builtin sets: the extender copied `digits` by analogy.
 - Rename the names that mislead: `table.pinnedRow` is the row node, not a pinned row;