* same notion of equivalence.
     *
     * @return a reference to {@code set}, for convenience
     */
    // Note: S should logically extend Set<? super E> but can't due to either
    // some javac bug or some weirdness in the spec, not sure which.
    @CanIgnoreReturnValue
    public <S extends Set<E>> S copyInto(S set) {
      set.addAll(this);