(defun c:linieumkehr ()
  (setq sset (ssget))
  (setq num (sslength sset) itm 0)
  (while (< itm num)
    (setq hnd (ssname sset itm))
    (setq ent (entget hnd))
    (if (= (cdr (assoc 0 ent)) "LINE")
      (progn
        (setq pt1 (cdr (assoc 10 ent)))
        (setq pt2 (cdr (assoc 11 ent)))
        (setq ent (subst (cons 10 pt2)(assoc 10 ent) ent))
        (setq ent (subst (cons 11 pt1)(assoc 11 ent) ent))
        (entmod ent)
      )
    )
    (setq itm (1+ itm))
  )
  (princ)
)