http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Laemmel-Advanced-Functional-Programming-The-Expression-Problem Better solution for the slide on 38 minute

PrettyPrinterWithNeg.cs

public class PrettyPrinterWithNeq : PrettyPrinter
{
	public override string PrettyPrint(Expr that)
	{
		var n = that as Neg;
		if (n != null) return "-(" + PrettyPrint(n.operand) + ")";
		
		return base.PrettyPrint(that);
	}
}