docs: public void addQuad(final String subject, final String predicate, final String object,
              String graph) {
          if (graph == null) {
              graph = "@default";
          }
          if (!containsKey(graph)) {
              put(graph, new ArrayList<Quad>());
          }
          ((ArrayList<Quad>) get(graph)).add(new Quad(subject, predicate, object, graph));
      }