*
   * {@snippet :
   * for (B b0 : sets.get(0)) {
   *   for (B b1 : sets.get(1)) {
   *     ...
   *     ImmutableList<B> tuple = ImmutableList.of(b0, b1, ...);
   *     // operate on tuple
   *   }
   * }
   * }
   *
   * <p>Note that if any input set is empty, the Cartesian product will also be empty. If no sets at