public ITextMarker Create(int startOffset, int length)
		{
			TextMarker m = new TextMarker(this, startOffset, length);
			markers.Add(m);
			// no need to mark segment for redraw: the text marker is invisible until a property is set
			return m;
		}
		internal void Remove(TextMarker marker)
		{
			markers.Remove(marker);
			Redraw(marker);
		}
 internal void Remove(TextMarker marker)
 {
     markers.Remove(marker);
     Redraw(marker);
 }