code: function(bbox) {
          var a = +bbox[0][0], b = +bbox[0][1], c = +bbox[1][0], d = +bbox[1][1];
          return [ Math.min(a, c), Math.min(b, d), Math.max(a, c), Math.max(b, d) ];
      }
docs: This method transforms a bounding box into a key for RTree index.