Undoing a move or a stretch command for an arc makes it all weird.
This is caused because the move and stretch commands are related to points instances but the arc changes its points representations.
Must be fixed but not such an easy task.
The whole moving point system must be restructured to keep reference to the kind of points for each element that it needs to change. It would be SO easy if we had blocks. :(
Adjusted the arc to follow the rule: The points that represent it will always be the same instances to allow operations on those instances to be performed. Therefore, the internal representation only adjusts the saved instances.
Oh! Forgot to say it is fixed on revision 1560.