def half_circle(center, options)
      case options[:side]
      when :left
        start_angle = 90
        end_angle = 270
      when :right
        start_angle = 270
        end_angle = 90
      end
      pie_slice(center,
                :radius => options[:radius],
                :start_angle => start_angle,
                :end_angle => end_angle,
                :stroke_both_sides => options[:stroke_both_sides])
    end
def half_circle(center, options)
      case options[:side]
      when :left
        start_angle = 90
        end_angle = 270
      when :right
        start_angle = 270
        end_angle = 90
      end
      pie_slice(center,
                :radius => options[:radius],
                :start_angle => start_angle,
                :end_angle => end_angle,
                :stroke_both_sides => options[:stroke_both_sides])
    end
options must include :radius and :side
 side is either :left or :right
 see pie_slice for explanation of optional
 :stroke_both_sides option