I tested, and it is 0.
angle(1,1,1,1) = 0
angle(1,1,2,1) = 0
angle(1,1,3,2) = 26.56505117707799
I guess I must write an event to prevent or change it from 0 if points are the same.
I was going to use it for a joystick for movement, so having it on the same point is likely to happen.