[Oberon] SystemV - Viewer Scroll

[Andreas Pirklbauer]

   > Hi Andreas,
   > Thank you, looking into your code. I wish, however to understand the original first. 

Please carefully read chapter 5.3, starting at page 66 of

   http://www.inf.ethz.ch/personal/wirth/ProjectOberon/PO.System.pdf

PS: When reading the code of TextFrames.Mod, it is helpful to visualize
the data structure that describes the currently displayed lines (F.trailer).

-ap