PROGRAM circle
LET r = 1                         ! radius of circle
CALL compute_aspect_ratio(r,xwin,ywin)
SET WINDOW -xwin,xwin,-ywin,ywin
SET COLOR "blue"
BOX CIRCLE -r,r,-r,r              ! draw circle
END

SUB compute_aspect_ratio(r,x,y)
    LET m = 0.1*r                 ! margin
    LET size = r + m
    ! px, py: # pixels in horizonal and vertical direction
    ASK PIXELS px,py
    IF px > py then
       LET aspect_ratio = px/py
       LET x = aspect_ratio*size
       LET y = size
    ELSE
       LET aspect_ratio = py/px
       LET x = size
       LET y = aspect_ratio*size
    END IF
END SUB