Receiver (Document):?

• The Document class represents the receiver of the command. It contains the

actual operations for adding and removing text from the document. The

AddText and RemoveText methods manipulate the internal content (a

StringBuilder).

public class Document

Follow:

private readonly StringBuilder _content = new StringBuilder();

public void AddText(string text) => _content.Append(text);

public void RemoveText(string text) =>

_content.Remove(_content.Length - text.Length, text.Length);

public override string ToString() => _content.ToString();