// Find the starting element
Tuple head = null;
TupleSet right = b.project(1);
for(Tuple x: u) if (!right.contains(x)) {head = x; break;}
if (head==null) return null;
final TupleFactory f = head.universe().factory();
// Form the list
list.add(head);
while(true) {
// Find head.next
Tuple headnext = null;