Spatial indexing is based on code by Marios Hadjieleftheriou, who has
developed spatial index library and published it under GNU GPL.

This code is based on version 1.0 of the library which can be found here:
http://u-foria.org/marioh/spatialindex/index.html

Spatial index library has Tools library as a dependency. Here we use it
in version 0.51b, it's available online (also under GNU GPL):
http://u-foria.org/marioh/tools/index.html

There are only few differences from the original libraries, namely:
- removed MVR-tree, TPR-tree and their variants of Point and Region
- removed SHA1 hash