A slightly different version line crossing, meets on any angle etc, but is very simple one at a time. Only rule do clockwise when picking lines, this implies where the arc is to go.
Code:
; draws an arc between 2 lines but does not erase
; by Alan H Aug 2015
(defun c:arcfill ( / pt1 pt2 pt3 pt4 obj1 obj2 obj3 obj4 obj5 obj6 pt5)
(alert "Pick in clockwise direction")
(setq oldsnap (getvar 'osmode))
(setvar 'Osmode 512)
(setq pt1 (getpoint "\npick 1st line"))
(setq obj1 (ssname (ssget pt1) 0))
(setq obj5 (vlax-ename->vla-object obj1)) ; convert to vl
(setq pt2 (getpoint "\npick 2nd line"))
(setq obj2 (ssname (ssget pt2) 0))
(setq obj6 (vlax-ename->vla-object obj2)) ; convert to vl
(setq rad (getdist "\nEter radius"))
(command "offset" rad Obj1 pt2 "")
(setq obj3 (vlax-ename->vla-object (entlast)))
(command "offset" rad Obj2 pt1 "")
(setq obj4 (vlax-ename->vla-object (entlast)))
(setq pt5 (vlax-invoke obj3 'intersectWith obj4 acExtendThisEntity)) ; centre pt
(setq pt3 (vlax-curve-getClosestPointto obj6 pt5))
(setq pt4 (vlax-curve-getClosestPointto obj5 pt5))
(command "arc" "C" pt5 pt3 pt4)
(vla-delete obj3)
(vla-delete obj4)
(setvar 'osmode oldsnap)
)
(c:arcfill)l