code: func (m *Machine) Enable() {
  	if !m.State.Enabled {
  		m.State = State{
  			Enabled:    true,
  			Generation: m.nextGen(),
  			LastRewind: m.Now,
  		}
  		m.scheduleTick(m.Now)
  	}
  }
docs: // Enable makes the cron machine start counting time.
  //
  // Does nothing if already enabled.