code: public GeospatialIndex geoElementPair(Element parent, Element lat, Element lon) {
      return new GeoElementPairImpl(parent, lat, lon);
    }
docs: Identifies a parent element with child latitude and longitude elements
  to match with a geospatial query.
  @param parent    the parent of the element with the coordinates
  @param lat    the element with the latitude coordinate
  @param lon    the element with the longitude coordinate
  @return    the specification for the index on the geospatial coordinates