# File gtk/sample/gtk-demo/cairo-arc.rb, line 25
    def draw(cr)
      cr.arc(CENTER_X, CENTER_Y, RADIUS, ANGLE1, ANGLE2)
      cr.stroke
      
      # draw helping lines
      cr.set_source_rgba(1, 0.2, 0.2, 0.6)
      cr.arc(CENTER_X, CENTER_Y, 0.05, 0, 2 * Math::PI)
      cr.fill
      cr.set_line_width(0.03)
      cr.arc(CENTER_X, CENTER_Y, RADIUS, ANGLE1, ANGLE1)
      cr.line_to(CENTER_X, CENTER_Y)
      cr.arc(CENTER_X, CENTER_Y, RADIUS, ANGLE2, ANGLE2)
      cr.line_to(CENTER_X, CENTER_Y)
      cr.stroke
    end