/// <summary>
/// Creates a substring of this rich text.
/// </summary>
public RichText Substring(int offset, int length)
{
if (offset == 0 && length == this.Length)
{
return(this);
}
string newText = text.Substring(offset, length);
RichTextModel model = ToRichTextModel();
OffsetChangeMap map = new OffsetChangeMap(2);
map.Add(new OffsetChangeMapEntry(offset + length, text.Length - offset - length, 0));
map.Add(new OffsetChangeMapEntry(0, offset, 0));
model.UpdateOffsets(map);
return(new RichText(newText, model));
}