Index: map_2nets.c
===================================================================
--- map_2nets.c	(revision 38739)
+++ map_2nets.c	(revision 38740)
@@ -63,6 +63,9 @@
 	if (seg == NULL)
 		return;
 
+	if (seg->junc_at[0] != NULL)
+		map->junc_at = seg->junc_at[0];
+
 	ns = calloc(sizeof(pcb_2netmap_iseg_t), 1);
 	if (!seg->has_invalid_hub) {
 		ns->next = map->isegs;
@@ -173,6 +176,7 @@
 	ec.cfg_prefer_term = 1;
 
 	map->ec = &ec;
+	map->junc_at = NULL;
 
 	htpp_init(&map->o2n, ptrhash, ptrkeyeq);
 
Index: map_2nets.h
===================================================================
--- map_2nets.h	(revision 38739)
+++ map_2nets.h	(revision 38740)
@@ -74,6 +74,7 @@
 
 typedef struct pcb_2netmap_s {
 	pcb_2netmap_oseg_t *osegs; /* output: head of a singly linked list */
+	pcb_any_obj_t *junc_at;    /* last junction detected at - if not NULL, input is not proper and can not be sorted into clean 2-nets */
 	unsigned find_rats:1;      /* config: set to 1 if rats shall be included */
 	unsigned find_floating:1;  /* include free floating nets that are not connected to any terminal */