xfarc — fill a part of an ellipse
xfarc(x,y,w,h,a1,a2)
xfarc fills a part of an ellipse contained in the rectangle
(x,y,w,h)
(upper-left point, width, height), and in the sector defined by
the angle alpha1 and the angle alpha1+alpha2.
alpha1 and alpha2 are
given respectively by a1/64 degrees and a2/64 degrees.
This function uses the current color and user coordinates.