code: public void registerAggregator(String name, Aggregator<?> aggregator) {
  		if (name == null || aggregator == null) {
  			throw new IllegalArgumentException("Name and aggregator must not be null");
  		}
  		if (this.registry.containsKey(name)) {
  			throw new RuntimeException("An aggregator is already registered under the given name.");
  		}
  		this.registry.put(name, aggregator);
  	}
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