code: static Set<String> serializeComparators(Criteria criteria, Configuration conf,
  	    List<String> comparatorRefs, List<String> comparatorInstanceFiles, String prefix)
  	    throws TupleMRException {
  
  		Set<String> instanceFiles = new HashSet<String>();
  		
  		if(criteria == null) {
  			return instanceFiles;
  		}
  
  		for(SortElement element : criteria.getElements()) {
  			if(element.getCustomComparator() != null) {
  				RawComparator<?> comparator = element.getCustomComparator();
  
  				if(!(comparator instanceof Serializable)) {
  					throw new TupleMRException("The class '" + comparator.getClass().getName()
  					    + "' is not Serializable."
  					    + " The customs comparators must implement Serializable.");
  				}
  
  				String ref = prefix + "|" + element.getName();
  				String uniqueName = UUID.randomUUID().toString() + '.' + "comparator.dat";
  				try {
  					InstancesDistributor.distribute(comparator, uniqueName, conf);
  					instanceFiles.add(uniqueName);
  				} catch(Exception e) {
  					throw new TupleMRException("The class " + comparator.getClass().getName() + 
  							" can't be serialized"
  							,e);
  				}
  
  				comparatorRefs.add(ref);
  				comparatorInstanceFiles.add(uniqueName);
  			}
  		}
  		
  		return instanceFiles;
  	}
docs: Returns the instance files created