code: func Interval(duration time.Duration) Expression {
  	negative := false
  	if duration < 0 {
  		negative = true
  		duration = -duration
  	}
  	return &intervalExpression{
  		duration: duration,
  		negative: negative,
  	}
  }
docs: // Interval returns a representation of duration
  // in a form "INTERVAL `hour:min:sec:microsec` HOUR_MICROSECOND"