Index: trunk/src/libgrbs/collision.c
===================================================================
--- trunk/src/libgrbs/collision.c	(revision 1330)
+++ trunk/src/libgrbs/collision.c	(revision 1331)
@@ -16,14 +16,14 @@
 	return 1;
 }
 
-static int coll_check_arc(grbs_t *grbs, grbs_2net_t *tn, grbs_arc_t *arc)
+static grbs_2net_t *coll_check_arc(grbs_t *grbs, grbs_2net_t *tn, grbs_arc_t *arc)
 {
 	if (!grbs_arc_is_outmost(grbs, arc)) /* do not check collision on inner arcs */
-		return 0;
+		return NULL;
 
 #warning TODO: callback: use ->new_* if exist or plain fields
 
-	return 0;
+	return NULL;
 }
 
 static void coll_report(grbs_t *grbs, grbs_2net_t *tn, grbs_2net_t *coll_tn, grbs_arc_t *coll_arc)
@@ -38,3 +38,8 @@
 	coll_report(grbs, tn, grbs_arc_parent_2net(coll_arc), coll_arc);
 }
 
+static void coll_report_tn(grbs_t *grbs, grbs_2net_t *tn, grbs_2net_t *coll_tn)
+{
+	if (grbs->coll_report_cb != NULL)
+		grbs->coll_report_cb(grbs, tn, coll_tn, NULL);
+}
Index: trunk/src/libgrbs/route_next.c
===================================================================
--- trunk/src/libgrbs/route_next.c	(revision 1330)
+++ trunk/src/libgrbs/route_next.c	(revision 1331)
@@ -68,6 +68,7 @@
 static grbs_addr_t *grbs_implement_incident(grbs_t *grbs, grbs_2net_t *tn, grbs_addr_t *from, double from_r, double from_a, grbs_point_t *to_pt, int segi, int new_at_from, grbs_addr_type_t curving, int concave_dir)
 {
 	grbs_point_t *from_pt = addr_point(from);
+	grbs_2net_t *coll_tn;
 
 	if (new_at_from) {
 		/* when creating a new go-around arc, we first place start (new_sa), so
@@ -80,7 +81,10 @@
 		res->obj.arc->new_da = 0;
 		res->obj.arc->new_adir = (curving == ADDR_ARC_CONCAVE) ? concave_dir : grbs_get_adir(to_pt->x, to_pt->y, from_pt->x, from_pt->y, from_r, from_a);
 		res->obj.arc->new_in_use = 1;
-		if (coll_check_arc(grbs, tn, res->obj.arc)) {
+		coll_tn = coll_check_arc(grbs, tn, res->obj.arc);
+		if (coll_tn != NULL) {
+			failprintf("arc collision failed\n");
+			coll_report_tn(grbs, tn, coll_tn);
 			grbs_arc_free(grbs, res->obj.arc);
 			grbs_addr_free_last(grbs);
 			return NULL;
@@ -102,6 +106,7 @@
 {
 	grbs_point_t *from_pt = addr_point(from), *to_pt;
 	grbs_arc_t *to_arc;
+	grbs_2net_t *coll_tn;
 
 	assert((from->type & ADDR_ARC_CONVEX) || (from->type & ADDR_ARC_CONCAVE));
 	assert(from->type & ADDR_ARC_END);
@@ -122,7 +127,10 @@
 	to_arc->new_adir = ((to->type & 0x0F) == ADDR_ARC_CONVEX) ? grbs_get_adir(from_pt->x, from_pt->y, to_pt->x, to_pt->y, to_r, to_a) : to_concave_dir;
 	to_arc->new_in_use = 1;
 
-	if (coll_check_arc(grbs, tn, to_arc)) {
+	coll_tn = coll_check_arc(grbs, tn, to_arc);
+	if (coll_tn != NULL) {
+		failprintf("arc collision failed\n");
+		coll_report_tn(grbs, tn, coll_tn);
 		to_arc->new_in_use = 0;
 		return NULL;
 	}
@@ -373,6 +381,7 @@
 	grbs_arc_t *from_under_me = NULL, *from_above_me = NULL;
 	grbs_arc_t *to_under_me = NULL, *to_above_me = NULL;
 	grbs_arc_t *arc, *to_arc, *tou;
+	grbs_2net_t *coll_tn;
 	double from_ang, to_ang, to_r, to_d;
 	int swapped = 0;
 
@@ -509,8 +518,12 @@
 		}
 	}
 
-	if (coll_check_arc(grbs, tn, from->obj.arc))
+	coll_tn = coll_check_arc(grbs, tn, from->obj.arc);
+	if (coll_tn != NULL) {
+		failprintf("arc collision failed\n");
+		coll_report_tn(grbs, tn, coll_tn);
 		return NULL;
+	}
 
 	to_arc->segi = arc->segi;
 	to_arc->new_r = to_r;