Michael,
I'm getting a much better understanding of arcs in Moi, now. But I am puzzled by this behaviour.

Here is the starting arc.



In this next image, I add a point snapped to the midpoint of an arc, and it adds another control point instead and keeps the arc intact.



In this third image, I trim the arc using the orange line and it splits the arc, but the top half has only one control point while the bottom has two.



>Len