anchor.diff:  
   specialValue
       : 'this' 
  -    | 'super' 
  -    | 'null'
  -    | 'none'
  +    | 'super'
       ;
   
   enumeration
  
positive.code:     | 'abstract'
      | 'default'
      | 'override'
      | 'optional'
      | 'mutable'
      | 'extension'
      | 'volatile'
  
negative.code:     ;
  
  type
      : typeNameWithArguments ('.' typeNameWithArguments)* abbreviation*
      -> ^(TYPE typeNameWithArguments+ abbreviation*)
      | 'subtype' abbreviation*
      -> ^(TYPE 'subtype' abbreviation*)
      ;
  
  abbreviation
      : '[' ']' | '?'
      ;
  
  typeNameWithArguments
  