draw a part of an ellipse
xarc(x,y,w,h,a1,a2)
:x,y,w,h four real values defining a rectangle. : :a1,a2 real values defining a sector. :
xarc draws 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 graphics color and user coordinates.
// isoview scaling
`plot2d`_(0,0,-1,"031"," ",[-2,-2,2,2])
`xset`_("color",3)
xarc(-1,1,2,2,0,90*64)
xarc(-1.5,1.5,3,3,0,360*64)