public static void Main(string[] args)
{
Document doc = new Document();
DocumentWriter writer = new DocumentWriter(doc);
// Write to the document:
writer.Write("Hello world!");
writer.Backspace();
doc.PrintDocument(); // Hello world
writer.Undo();
doc.PrintDocument(); // Hello world!
writer.Redo();
doc.PrintDocument(); // Hello world
writer.Write(" Goodbye!");
doc.PrintDocument(); // Hello world Goodbye!
writer.Undo();
doc.PrintDocument(); // Hello world
writer.Undo();
doc.PrintDocument(); // Hello world!
}