docs: public void addQuad(final String s, final String p, final String value, final String datatype,
              final String language, String graph) {
          if (graph == null) {
              graph = "@default";
          }
          if (!containsKey(graph)) {
              put(graph, new ArrayList<Quad>());
          }
          ((ArrayList<Quad>) get(graph)).add(new Quad(s, p, value, datatype, language, graph));
      }