code: @Override
      public Iterator<String> getKeys(String prefix)
      {
          Set<String> keys = new LinkedHashSet<String>();
          for (Iterator<String> it = overrideProperties.getKeys(prefix); it.hasNext();) {
              keys.add(it.next());
          }
          for (Configuration config : configList)
          {
              for (Iterator<String> it = config.getKeys(prefix); it.hasNext();)
              {
                  keys.add(it.next());
              }
          }
  
          return keys.iterator();
      }
docs: Get the list of the keys contained in the sub configurations that match the
  specified prefix.