public class PrefixExpressionPrinter : IExpressionVisitor
{
    StringBuilder sb;
    public PrefixExpressionPrinter(StringBuilder sb)
    {
        this.sb = sb;
    }
    public void Visit(Literal literal)
    {
        sb.Append(literal.Value);
    }
    public void Visit(Addition addition)
    {
        sb.Append("+ ");
        addition.Left.Accept(this);
        sb.Append(" ");
        addition.Right.Accept(this);
    }
}