@Override
 public void map(final Vertex vertex, final MapEmitter<Object, Double> emitter) {
   final Property pageRank = vertex.property(PageRankVertexProgram.PAGE_RANK);
   if (pageRank.isPresent()) {
     emitter.emit(vertex.id(), (Double) pageRank.value());
   }
 }
 public static void validatePropertyEquality(
     final Property originalProperty, final Property otherProperty) {
   assertEquals(originalProperty, otherProperty);
   assertEquals(otherProperty, originalProperty);
   if (originalProperty.isPresent()) {
     assertEquals(originalProperty.key(), otherProperty.key());
     assertEquals(originalProperty.value(), otherProperty.value());
     assertEquals(originalProperty.element(), otherProperty.element());
   }
 }