Add the with_additional_node function to the ContextTree.

src/parser/list.rs

@@ -23,7 +23,7 @@ impl<T> List<T> {
         }
     }
 
-    pub fn get_data(&self) -> &T {
-        &self.data
+    pub fn get_data(&self) -> Option<&T> {
+        self.head.map(|rc_node| &rc_node.as_ref().data)
     }
 }