code: function rbushTreeItem(ring, edge) {
  
      var start = coord[ring][edge];
      var end = coord[ring][edge+1];
  
      if (start[0] < end[0]) {
        var minX = start[0], maxX = end[0];
      } else {
        var minX = end[0], maxX = start[0];
      };
      if (start[1] < end[1]) {
        var minY = start[1], maxY = end[1];
      } else {
        var minY = end[1], maxY = start[1];
      }
      return {minX: minX, minY: minY, maxX: maxX, maxY: maxY, ring: ring, edge: edge};
    }
docs: Function to return a rbush tree item given an ring and edge number