code: public GeospatialIndex geoJSONPropertyPair(JSONProperty parent, JSONProperty lat, JSONProperty lon) {
      if ( parent == null ) throw new IllegalArgumentException("parent cannot be null");
      if ( lat == null )    throw new IllegalArgumentException("lat cannot be null");
      if ( lon == null )    throw new IllegalArgumentException("lon cannot be null");
      return new GeoJSONPropertyPairImpl(parent, lat, lon);
    }
docs: Identifies a parent json property with child latitude and longitude json properties
  to match with a geospatial query.
  @param parent    the parent json property of lat and lon
  @param lat    the json property with the latitude coordinate
  @param lon    the json property with the longitude coordinate
  @return    the specification for the index on the geospatial coordinates