code: public synchronized void appendToListProp(String key, String value) {
      if (contains(key)) {
        setProp(key, LIST_JOINER.join(getProp(key), value));
      } else {
        setProp(key, value);
      }
    }
docs: Appends the input value to a list property that can be retrieved with {@link #getPropAsList}.
  
  <p>
  List 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).