code: func (m *Machine) rewindIfNecessary(now time.Time) {
  	if m.State.Enabled && m.State.LastTick.When.IsZero() {
  		m.State.LastRewind = now
  		m.State.Generation = m.nextGen()
  		m.scheduleTick(now)
  	}
  }
docs: // rewindIfNecessary implements RewindIfNecessary, accepting corrected 'now'.
  //
  // This is important for OnTimerTick. Note that m.Now is part of inputs and must
  // not be mutated.