Workflow: check file imports

.github/workflows/check_file_import.yml

@@ -0,0 +1,22 @@
+on: 
+  push: 
+
+name: check file import
+
+jobs: 
+  check_file_import: 
+    name: check file import 
+    runs-on: ubuntu-latest
+    steps: 
+     - name: clean up
+       run: |
+        find . -name . -o -prune -exec rm -rf -- {} +
+
+    - uses: actions/checkout@v4
+  
+    - name: update HepLean.lean 
+        run: |
+          git ls-files 'HepLean/*.lean' | LC_ALL=C sort | sed 's/\.lean//;s,/,.,g;s/^/import /' > HepLean.lean
+
+    - name: check that all files are imported
+        run: git diff --exit-code