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