code: public <NV> Graph<K, NV, EV> mapVertices(final MapFunction<Vertex<K, VV>, NV> mapper, TypeInformation<Vertex<K, NV>> returnType) {
  		DataSet<Vertex<K, NV>> mappedVertices = vertices.map(
  				new MapFunction<Vertex<K, VV>, Vertex<K, NV>>() {
  					private Vertex<K, NV> output = new Vertex<>();
  
  					public Vertex<K, NV> map(Vertex<K, VV> value) throws Exception {
  						output.f0 = value.f0;
  						output.f1 = mapper.map(value);
  						return output;
  					}
  				})
  				.returns(returnType)
  				.withForwardedFields("f0")
  					.name("Map vertices");
  
  		return new Graph<>(mappedVertices, this.edges, this.context);
  	}
docs: Apply a function to the attribute of each vertex in the graph.
  
  @param mapper the map function to apply.
  @param returnType the explicit return type.
  @return a new graph