 placed by hitting the <kbd>Shift-C</kbd> key. First click the start point, then the end point. 
   Moving the mouse will show the arc passing thru the 2 points and the mouse waypoint. 
   Clicking will place the arc.
   Arcs may be modified after creation by selecting in stretch mode (<kbd> Ctrl-Button1-drag </kbd>) one of the arc ends
   or the arc center:<br>
   - (end point selected in stretch mode): by starting a move (<kbd>m</kbd>) 
      operation and moving the mouse the arc sweep may be changed.<br