Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
Halve the ATK of your opponent's attacking monsters. Once per turn, during your Standby Phase, pay 2000 LP or destroy this card.
--銀幕の鏡壁
--Mirror Wall
local s,id=GetID()
function s.initial_effect(c)
	--activate
	local e1=Effect.CreateEffect(c)
	e1:SetType(EFFECT_TYPE_ACTIVATE)
	e1:SetCategory(CATEGORY_ATKCHANGE)
	e1:SetProperty(EFFECT_FLAG_DAMAGE_STEP)
	e1:SetCode(EVENT_FREE_CHAIN)
	e1:SetHintTiming(0,TIMING_DAMAGE_STEP)
	e1:SetCondition(aux.StatChangeDamageStepCondition)
	e1:SetTarget(s.target)
	e1:SetOperation(s.operation)
	c:RegisterEffect(e1)
	--
	local e2=Effect.CreateEffect(c)
	e2:SetType(EFFECT_TYPE_FIELD+EFFECT_TYPE_CONTINUOUS)
	e2:SetCode(EVENT_ATTACK_ANNOUNCE)
	e2:SetRange(LOCATION_SZONE)
	e2:SetOperation(s.operation)
	c:RegisterEffect(e2)
	--atkchange
	local e3=Effect.CreateEffect(c)
	e3:SetType(EFFECT_TYPE_FIELD)
	e3:SetCode(EFFECT_SET_ATTACK_FINAL)
	e3:SetRange(LOCATION_SZONE)
	e3:SetTargetRange(0,LOCATION_MZONE)
	e3:SetTarget(s.atktg)
	e3:SetValue(s.atkval)
	c:RegisterEffect(e3)
	local g=Group.CreateGroup()
	g:KeepAlive()
	e1:SetLabelObject(g)
	e2:SetLabelObject(g)
	e3:SetLabelObject(g)
	--maintain
	local e4=Effect.CreateEffect(c)
	e4:SetType(EFFECT_TYPE_FIELD+EFFECT_TYPE_CONTINUOUS)
	e4:SetProperty(EFFECT_FLAG_CANNOT_DISABLE+EFFECT_FLAG_UNCOPYABLE)
	e4:SetCode(EVENT_PHASE|PHASE_STANDBY)
	e4:SetRange(LOCATION_SZONE)
	e4:SetCountLimit(1)
	e4:SetCondition(s.mtcon)
	e4:SetOperation(s.mtop)
	c:RegisterEffect(e4)
end
function s.target(e,tp,eg,ep,ev,re,r,rp,chk)
	if chk==0 then return true end
	e:GetLabelObject():Clear()
end
function s.operation(e,tp,eg,ep,ev,re,r,rp)
	local a=Duel.GetAttacker()
	if a and a:IsControler(1-tp) and a:IsFaceup() and a:IsLocation(LOCATION_MZONE) then
		e:GetLabelObject():AddCard(a)
		if a:GetFlagEffect(id)==0 then
			a:RegisterFlagEffect(id,RESET_EVENT|RESETS_STANDARD,0,1)
		end
	end
end
function s.atktg(e,c)
	return c:GetFlagEffect(id)~=0 and e:GetLabelObject():IsContains(c)
end
function s.atkval(e,c)
	return c:GetAttack()/2
end
function s.mtcon(e,tp,eg,ep,ev,re,r,rp)
	return Duel.IsTurnPlayer(tp)
end
function s.mtop(e,tp,eg,ep,ev,re,r,rp)
	if Duel.CheckLPCost(tp,2000) and Duel.SelectYesNo(tp,aux.Stringid(id,0)) then
		Duel.PayLPCost(tp,2000)
	else
		Duel.Destroy(e:GetHandler(),REASON_COST)
	end
end