if i try and use the arc, pieslice or ellipse methods of a Draw object with some sets of coordinates no error is raised but nothing is drawn either.
this occurs when one or both axis coordinates decreases between the first and second coord of the bounding box.
eg:

draw.arc((0,0,10,10), 0, 90, 'red') # works
draw.arc((10,0,0,10), 0, 90, 'red') # doesn't work
draw.arc((0,10,10,0), 0, 90, 'red') # doesn't work
draw.arc((10,10,0,0), 0, 90, 'red') # doesn't work

I'm using python3.4 and pillow 5.0.0

I have no issue with the API requiring the coordinates in the correct order, but I couldn't find any documentation stating that it was required, and obviously no error was raised either.
I think it needs documenting and maybe raise an error too?