for tier, st := range other.Tiers {
		ats.Tiers[tier] = ats.Tiers[tier].add(st)
	}
}

func (ats *allTierStats) clone() *allTierStats {
	if ats == nil {
		return nil
	}
	dst := *ats
	dst.Tiers = make(map[string]tierStats, len(ats.Tiers))
	for tier, st := range ats.Tiers {
		dst.Tiers[tier] = st
	}
	return &dst
}