code: public void registerAggregator(String name, Class<? extends Aggregator<?>> aggregator) {
  		this.aggregators.put(name, aggregator);
  	}
docs: Registers a new aggregator. Aggregators registered here are available during the execution of the vertex updates
  via {@link VertexUpdateFunction#getIterationAggregator(String)} and
  {@link VertexUpdateFunction#getPreviousIterationAggregate(String)}.
  
  @param name The name of the aggregator, used to retrieve it and its aggregates during execution.
  @param aggregator The aggregator.