* equivalent to regular Java division, {@code p / q}; and if it is {@link RoundingMode#FLOOR},
   * then this method is equivalent to {@link Math#floorDiv(long,long) Math.floorDiv}{@code (p, q)}.
   *
   * @throws ArithmeticException if {@code q == 0}, or if {@code mode == UNNECESSARY} and {@code a}
   *     is not an integer multiple of {@code b}
   */
  @GwtIncompatible // TODO