static async Task Main(string[] args)
{
    //defining expression
    var addition = new Addition(
        new Addition(
            new Addition(
                new Literal(5.0), new Literal(6.0)),
            new Literal(20.0)),
        new Addition(
            new Literal(1.0), new Literal(3.0)));

    var sb = new StringBuilder();
    //creating visitor
    var expressionPrinter = new InfixExpressionPrinter(sb);
    //addition will be "visited" by our visitor
    addition.Accept(expressionPrinter);
    Console.WriteLine(sb);   //outputs (((5+6)+20)+(1+3))
    Console.ReadLine();
}