fixed odd orders

src/tree.py

@@ -306,13 +306,23 @@ class TreeWalker:
 
     def ensure_exercises(self, section_path):
         config = self.ensure_section_config(section_path)
+        flag = False
         for e in os.listdir(section_path):
             base, ext = os.path.splitext(e)
             _, source = os.path.split(e)
             if ext != ".md":
                 continue
-            meta_path = os.path.join(section_path, base + ".json")
+            mfile = base + ".json"
+            meta_path = os.path.join(section_path, mfile)
             self.ensure_exercises_meta(meta_path, source)
+            export = config.get("export", [])
+            if mfile not in export:
+                export.append(mfile)
+                flag = True
+                config["export"] = export
+
+        if flag:
+            dump_json(os.path.join(section_path, "config.json"), config, True, True)
 
         for e in config.get("export", []):
             full_name = os.path.join(section_path, e)