1. A document handling system comprising: a pen instrument to track writing motions made with the pen instrument so that an electronic document is modified by incorporating reproductions of strokes made with the pen instrument, wherein the pen instrument tracks its own motion and generates signals that represent the writing motions made with the pen instrument; and a processing unit having an interface to receive the signals from the pen instrument, wherein the processing unit stores the electronic document and modifies the electronic document in response to the strokes made with the pen instrument such that representations of the writing motions made with the pen instrument are displayed when the electronic document is displayed.
7. The system of claim 1 further comprising at least two infrared radiating scanning sources located at prescribed positions external to the pen instrument, the scanning sources to illuminate the pen instrument, wherein the pen instrument determines movements based, at least in part, on the scanning sources.