Implement ptr_eq on the context types.

src/parser/list.rs

@@ -64,4 +64,12 @@ impl<T: Debug> List<T> {
     pub fn is_empty(&self) -> bool {
         self.head.is_none()
     }
+
+    pub fn ptr_eq(&self, other: &List<T>) -> bool {
+        match (self.head.as_ref(), other.head.as_ref()) {
+            (None, None) => true,
+            (None, Some(_)) | (Some(_), None) => false,
+            (Some(me), Some(them)) => Rc::ptr_eq(me, them),
+        }
+    }
 }