identity function for slices that are already allocated in the heap.
And his latest book, Chaos in the Box: get it now
，这一点在同城约会中也有详细论述
Special Features
Works with Regional Maps: Download only the countries you need. HH-Routing seamlessly calculates routes across the borders of your downloaded map files (as long as they are compatible, see limitations). Clusters that overlap a region's boundary are included within that region's data.