docs: protected static int shrinkActiveSet(DBIDArrayIter ix, PointerHierarchyRepresentationBuilder builder, int end, int x) {
      if(x == end - 1) { // Can truncate active set.
        while(builder.isLinked(ix.seek(--end - 1))) {
          // Everything happens in while condition already.
        }
      }
      return end;
    }