code: private static Collection<JobIdWithStatus> combine(
  			Collection<JobIdWithStatus> first,
  			Collection<JobIdWithStatus> second) {
  		checkNotNull(first);
  		checkNotNull(second);
  
  		ArrayList<JobIdWithStatus> result = new ArrayList<>(first.size() + second.size());
  
  		result.addAll(first);
  		result.addAll(second);
  
  		return result;
  	}
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