anchor.diff:  directive
       : 'return' assignable? 
       | 'produce' assignable
  -    | 'throw' expression 
  +    | 'throw' expression? 
       | 'break' 
       | 'found'
       ;
  
positive.code: //special rule for syntactic predicates
  declarationStart
      :  declarationModifier 
      | ( userAnnotation annotations? )? (type|'assign'|'void') LIDENTIFIER
      ;
  
  
negative.code: 
  // Lexer
  
  NATURALLITERAL
      : ('0' .. '9')('0' .. '9')*
      | '\'' HexDigit HexDigit HexDigit HexDigit '\''
      | '\'' HexDigit HexDigit '\''
  