anchor.diff:   ;
   
   break_statement
  - : BREAK
  + : Mark=BREAK
    ;
   
   return_statement
  - : RETURN expression?
  + : Mark=RETURN Value=expression?
    ;
   
   compound_statement
  
positive.code: 
  bool_not_expression
   : comparison_expression
   | NOT Right=bool_not_expression
   ;
  
  comparison_expression
  
negative.code: 
  power_expression
   : // right-associative
     Left=unary_expression (Op=POWER Right=power_expression)?
   ;
  
  unary_expression
  