diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 8826d59..1a4b73b 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -26,7 +26,17 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 18
+      - run: git tag pre-updates
       - run: npm install
+      - name: Create generated files
+        run: |
+          npm run gen
+          git diff pre-updates --no-patch --exit-code; exitcode=$?
+          
+          if [ "$exitcode" -ne 0 ]; then
+            echo '::error::Generated files are out of date!'
+            echo '::error::run `npm run gen` and commit the changes'
+            exit "$exitcode"
+          fi
       - run: npm test
         # FIXME: also parse tests/*.just
-
diff --git a/grammar.js b/grammar.js
index d1cb09c..7bd5c47 100644
--- a/grammar.js
+++ b/grammar.js
@@ -80,7 +80,7 @@ module.exports = grammar({
       ),
 
     // boolean       : ':=' ('true' | 'false')
-    boolean: (_) => choice("true", "false"),
+    boolean: (_) => choice("true", "false", "a"),
 
     // expression    : 'if' condition '{' expression '}' 'else' '{' expression '}'
     //               | value '/' expression