keys:

         l = load
         s = save
         o = organize

special keys:

    delete = delete selected node
      home = select first node
       end = select last node
  up-arrow = select previous node
down-arrow = select next node

mouse buttons:

         1 = make a node
         2 = draw an arrow
   shift-1 = move selected node
   shift-2 = move head or tail of arrow

