anchor.diff:  
   LOCAL
       :   'local'
  -    ;
  -
  -NONE
  -    :   'none'
  -    ;
  -
  -NULL
  -    :   'null'
       ;
   
   NONEMPTY
  
positive.code:     | 'abstract'
      | 'default'
      | 'override'
      | 'optional'
      | 'mutable'
      | 'extension'
      | 'volatile'
  
negative.code: 
  specialValue
      : 'this' 
      | 'super'
      ;
  
  enumeration
  