+void Zone::set_preferred_direction(Direction d)
+{
+ if(up_end<0)
+ throw logic_error("no direction");
+
+ preferred_dir = d;
+
+ if(preferred_dir)
+ {
+ TrackIter iter = get_end(preferred_dir).reverse();
+ while(iter && tracks.count(iter.track()))
+ {
+ iter->set_preferred_exit(iter.entry());
+ iter = next_iter(iter);
+ }
+ }
+ else
+ {
+ for(TrackSet::iterator i=tracks.begin(); i!=tracks.end(); ++i)
+ (*i)->set_preferred_exit(-1);
+ }
+}
+