How to use the rtree.core function in Rtree

pvt_offset = 0

    mrf_dom = build_mrf_dom(tile_matrices, target_extents, tile_size, proj)
    with open(os.path.join(output_path, mrf_prefix) + '.mrf', 'w+') as f:
        f.write(mrf_dom.toprettyxml())

    spatial_dbs = []
    source_schemas = []
    # Dump contents of shapefile into a mutable rtree spatial database for faster searching.
    for input_file in input_file_path:
        print 'Processing ' + input_file
        with fiona.open(input_file) as shapefile:
            try:
                spatial_db = rtree.index.Index(
                    rtree_index_generator(list(shapefile), filter_list))
            except rtree.core.RTreeError as e:
                print 'ERROR -- problem importing feature data. If you have filters configured, the source dataset may have no features that pass. Err: {0}'.format(
                    e)
                sys.exit()
            spatial_dbs.append(spatial_db)
            source_schema = shapefile.schema['geometry']
            source_schemas.append(source_schema)
            if debug:
                print 'Points to process: ' + str(
                    spatial_db.count(spatial_db.bounds))

    # Build tilematrix pyramid from the bottom (highest zoom) up. We generate tiles left-right, top-bottom and write them
    # successively to the MRF.
    for i, tile_matrix in enumerate(reversed(tile_matrices)):
        z = len(tile_matrices) - i - 1

        for idx, spatial_db in enumerate(spatial_dbs):