A small question regarding the DNF search algorithm in the paper

[csimplestring]

Hey, glad to know you've done such a good job for implementing this algorithm for that paper.

When I read the paper, in the Complexity section at page 6, the author says: The sorting in Step 14 takes O(log(|S|)) because the inverted list is initially sorted, and we only need to bubble down one posting list in PLists using a heap for each posting list skipped. But I did not find a 'heap' is used to sort the posting lists in this implementation, could you please help me clarify this?

Thanks!