add: arc 3pts method
  I think that Start,End,Point method is more useful.
  - I usually know end point or I can find it with a construction lines.
  - When I pick start and end point, the third point influences only an arc radius while with current method the arc radius and point location are still unknown.
  - When I have three explicit points, there is no problem with picking end point first

So if Crusoe begs for an alteration, I join him!

Petr