anchor.diff:  
   === Reference implementation
   
  -* TBD
  +https://github.com/apache/groovy/pull/1606
   
   === JIRA issues
   
  
positive.code: * Provide checks in other places where such extension might occur implicitly, e.g.:&nbsp;with `@Delegate`,
  when using type coercion, etc.
  
  ==== Potential extensions
  
  The following potential extensions are possibly all desirable but
  
negative.code: 
  * Introduce the `sealed` modifier and `permits` clause in the grammar.
  
  * Require that all classes within a sealed hierarchy be compiled at the same time.
  
  * Require that all classes within a sealed hierarchy belong to the same JPMS module.
  
  * Add warnings to the static compiler if a switch is used for a sealed hierarchy
  and not all types are exhaustively covered.
  