The TWINHEAPISECT algorithm manages the lower
and upper bounds of the frontier intervals in two
separate heaps instead of a single heap as in the basic
scan-line algorithm. As a result, heap insertions are
more efficient than in the basic scan-line algorithm
since each heap is 50% smaller (so it takes less time to
adjust the heap structures when inserting an element).
Thus the TWINHEAPISECT algorithm is more efficient
than SCANLINEISECT, as will be confirmed by the
experiments in Section 5.