Key Observation: every subset of a frequent itemset is also frequent.
select p.item1, p.item2, …, p.itemk-1, q.itemk-1
where p.item1 = q.item1, …, p.itemk-2 = q.itemk-2,
Next, we delete all itemsets c in Ck such that some
(k-1)-subset of c is not in Lk.