Expression block - Gen<Expr> - 8 - (0:0,0)
    Transition span - Gen<None> - [@] - SpanEditHandler;Accepts:None - (0:0,0) - Tokens:1
        CSharpTokenType.Transition;[@];
    Code span - Gen<Expr> - [val?[0]] - ImplicitExpressionEditHandler;Accepts:NonWhiteSpace;ImplicitExpression[RTD];K14 - (1:0,1) - Tokens:5
        CSharpTokenType.Identifier;[val];
        CSharpTokenType.QuestionMark;[?];
        CSharpTokenType.LeftBracket;[[];
        CSharpTokenType.IntegerLiteral;[0];
        CSharpTokenType.RightBracket;[]];
