code: public synchronized void appendToSetProp(String key, String value) {
      Set<String> set = value == null ? Sets.<String>newHashSet() : Sets.newHashSet(LIST_SPLITTER.splitToList(value));
      if (contains(key)) {
        set.addAll(getPropAsSet(key));
      }
      setProp(key, LIST_JOINER.join(set));
    }
docs: Appends the input value to a set property that can be retrieved with {@link #getPropAsSet}.
  
  <p>
  Set properties are internally stored as comma separated strings. Adding a value that contains commas (for
  example "a,b,c") will essentially add multiple values to the property ("a", "b", and "c"). This is
  similar to the way that {@link org.apache.hadoop.conf.Configuration} works.
  </p>
  
  @param key property key
  @param value property value (if it includes commas, it will be split by the commas).