code: private HyperBoundingBox mbr(final int[] entries, final int from, final int to) {
      SpatialEntry first = this.node.getEntry(entries[from]);
      ModifiableHyperBoundingBox mbr = new ModifiableHyperBoundingBox(first);
      for(int i = from + 1; i < to; i++) {
        mbr.extend(this.node.getEntry(entries[i]));
      }
      return mbr;
    }
docs: Computes and returns the mbr of the specified nodes, only the nodes between
  from and to index are considered.
  
  @param entries the array of node indices
  @param from the start index
  @param to the end index
  @return the mbr of the specified nodes