帮我看看为什么圆弧不能延伸?

cjs

(defun c:test (/)
  (vl-load-com)
  (setq lastpl (car (entsel "\n pl:"))
           mobj   (car (entsel "\n obj:"))
   ) ;_ 结束setq
   (setq sp (vlax-curve-getstartpoint lastpl)
            ep (vlax-curve-getendpoint lastpl)
    )
     (command "extend"
      mobj
      ""
      "e"
      "e"
      (list lastpl ep)
      (list lastpl sp)
      ""
    ) ;_ 结束command
) ;_ 结束defun