private Collection<String> customer_airlines = new HashSet<String>();
public FrequentFlyerIterable(Table catalog_tbl, long num_customers) {
super(catalog_tbl, num_customers, new int[]{ 0, 1, 2 });
this.customer_id_iterator = new CustomerIdIterable(profile.airport_max_customer_id).iterator();
this.last_customer_id = this.customer_id_iterator.next();
// A customer is more likely to have a FREQUENTY_FLYER account with
// an airline that has more flights.
// IMPORTANT: Add one to all of the airlines so that we don't get trapped