code: protected final void append (
          ArrayList<String> list, int index, String value)
      {
          // expand the list as necessary
          while (list.size() <= index) {
              list.add("");
          }
          list.set(index, list.get(index) + value);
      }
docs: Appends the supplied string to the contents of the list at the supplied index. If the list
  has no contents at the supplied index, the supplied value becomes the contents at that
  index.