code: public static MutableClock of(Instant instant, ZoneId zone) {
          Objects.requireNonNull(instant, "instant");
          Objects.requireNonNull(zone, "zone");
          return new MutableClock(new InstantHolder(instant), zone);
      }
docs: Obtains a new {@code MutableClock} set to the specified instant,
  converting to date and time using the specified time-zone.
  
  @param instant the initial value for the clock, not null
  @param zone the time-zone to use, not null
  @return a new {@code MutableClock}, not null