anchor.diff:  
   NONEMPTY
       :   'nonempty'
  -    ;
  -
  -GET
  -    :   'get'
  -    ;
  -
  -SET
  -    :   'set'
       ;
   
   RETURN
  
positive.code:     | specialValue 
      | nonstringLiteral
      | prefixOperator
      | 'get'
      | 'set'
      ;
  
  expression
  
negative.code:     | base
      ;
  
  postfixOperator
      : '--' | '++'
      ;	
  