JOIN multiple lines to polylines JOINPOLY

FAQ: [23.2] How to JOIN multiple lines to polylines?

Simply try to join each element with all selected, but beware thata entity already joined cannot be entget'ed anymore, because it's deleted.

;;; This sample converts all selected elements to polylines and

;;; tries to join as much as possible.

(defun C:JOINPOLY (/ ele ss)
  (foreach ele (std-sslist (setq ss (ssget)))

(if (entget ele) ;not already joined

      (cond
        ((std-entity-type-p ele '("ARC" "LINE"))

;; in fact you should check Z of lines and UCS here too

(command "_PEDIT" ele "_Y" "_J" ss "" ""); convert+JOIN

        )
        ((and (std-entity-type-p ele '("POLYLINE" "LWPOLYLINE")) 

(not (std-flagsetp 1 ele)) ;not closed

(< (rem (std-getflag ele) 128) 8)) ;ignore meshes

(command "_PEDIT" ele "_J" ss "" "");ucs check omitted

        )
      )
    )
  )
)