Index: trunk/src/libgrbs/collision.c
===================================================================
--- trunk/src/libgrbs/collision.c	(revision 1316)
+++ trunk/src/libgrbs/collision.c	(revision 1317)
@@ -26,14 +26,15 @@
 	return 0;
 }
 
-static void coll_report(grbs_t *grbs, grbs_2net_t *tn, grbs_2net_t *coll_tn)
+static void coll_report(grbs_t *grbs, grbs_2net_t *tn, grbs_2net_t *coll_tn, grbs_arc_t *coll_arc)
 {
-#warning TODO: callback: use ->coll_report
+	if (grbs->coll_report_cb != NULL)
+		grbs->coll_report_cb(grbs, tn, coll_tn, coll_arc);
 }
 
 
 static void coll_report_arc(grbs_t *grbs, grbs_2net_t *tn, grbs_arc_t *coll_arc)
 {
-	coll_report(grbs, tn, grbs_arc_parent_2net(coll_arc));
+	coll_report(grbs, tn, grbs_arc_parent_2net(coll_arc), coll_arc);
 }
 
Index: trunk/src/libgrbs/grbs.h
===================================================================
--- trunk/src/libgrbs/grbs.h	(revision 1316)
+++ trunk/src/libgrbs/grbs.h	(revision 1317)
@@ -81,6 +81,9 @@
 struct grbs_s {
 	long uids;
 
+	/* user settable callbacks (optional) */
+	void (*coll_report_cb)(grbs_t *grbs, grbs_2net_t *tn, grbs_2net_t *coll_tn, grbs_arc_t *coll_arc); /* rounting tn happened to collide with coll_tn; if coll_arc is not NULL, the collision happened at that arc */
+
 	/* keep track on all objects so they can be freed or quickly reallocated */
 	gdl_list_t all_2nets, free_2nets;
 	gdl_list_t all_points, free_points;