update CHANGES

Author: fblanqui

CHANGES.md

@@ -3,6 +3,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/),
 and this project adheres to [Semantic Versioning](https://semver.org/).
 
+## 1.3.1
+
+### Fixed
+
+- declare theorems as opaque symbols
+
 ## 1.3.0
 
 ### Added