code: @Nullable
      public synchronized URI getSystemType() throws IllegalArgumentException {
          URI result = null;
          for (final URI type : get(RDF.TYPE, URI.class)) {
              if (type.getNamespace().equals(KS.NAMESPACE)) {
                  Preconditions.checkArgument(result == null, "Multiple system types: " + result
                          + ", " + type);
                  result = type;
              }
          }
          return result;
      }
docs: Returns the system type for this record, i.e., the {@code rdf:type} URI under the
  {@code ks:} namespace, if any.
  
  @return the system type or null if not set
  @throws IllegalArgumentException
  in case multiple system types are bound to the record