code: public int[] indices(Collection<E> elems) {
      int[] indices = new int[elems.size()];
      int i = 0;
      for (E elem : elems) {
        indices[i++] = indexOf(elem);
      }
      return indices;
    }
docs: Returns the index of each elem in a List.
  @param elems The list of items
  @return An array of indices