Expression block - Gen<Expr> - 9 - (0:0,0)
    Transition span - Gen<None> - [@] - SpanEditHandler;Accepts:None - (0:0,0) - Tokens:1
        SyntaxKind.Transition;[@];
    Code span - Gen<Expr> - [val?[-1]] - ImplicitExpressionEditHandler;Accepts:NonWhitespace;ImplicitExpression[RTD];K14 - (1:0,1) - Tokens:6
        SyntaxKind.Identifier;[val];
        SyntaxKind.QuestionMark;[?];
        SyntaxKind.LeftBracket;[[];
        SyntaxKind.Minus;[-];
        SyntaxKind.IntegerLiteral;[1];
        SyntaxKind.RightBracket;[]];
