anchor.diff:  
   * Introduce the `sealed` modifier and `permits` clause in the grammar.
   
  -* Support `non-sealed` or `unsealed` sub-hierarchies.
  -
   * 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 module.
  +* 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.
  
positive.code: 
  === Reference implementation
  
  * TBD
  
  === JIRA issues
  
  
negative.code: * Provide checks in other places where such extension might occur implicitly, e.g.:&nbsp;with `@Delegate`,
  when using type coercion, etc.
  
  * Support `non-sealed` or `unsealed` sub-hierarchies. (See JEP-409)
  
  * Allow the permitted subclasses to be inferred automatically just for the case
  where the base and all permitted subclasses are in the same file. (See JEP-409)
  
  ==== Potential extensions
  
  The following potential extensions are possibly all desirable but
  