Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
This card must attack if able. When this face-up Attack Position card is selected as an attack target, change it to Defense Position. Once per turn, this card cannot be destroyed by battle.
--カラクリ樽 真九六
--Karakuri Barrel mdl 96 "Shinkuro"
local s,id=GetID()
function s.initial_effect(c)
	--must attack
	local e1=Effect.CreateEffect(c)
	e1:SetType(EFFECT_TYPE_SINGLE)
	e1:SetCode(EFFECT_MUST_ATTACK)
	c:RegisterEffect(e1)
	--to defense
	local e3=Effect.CreateEffect(c)
	e3:SetDescription(aux.Stringid(id,0))
	e3:SetCategory(CATEGORY_POSITION)
	e3:SetType(EFFECT_TYPE_TRIGGER_F+EFFECT_TYPE_SINGLE)
	e3:SetCode(EVENT_BE_BATTLE_TARGET)
	e3:SetCondition(s.poscon)
	e3:SetOperation(s.posop)
	c:RegisterEffect(e3)
	--battle indes
	local e4=Effect.CreateEffect(c)
	e4:SetType(EFFECT_TYPE_SINGLE)
	e4:SetProperty(EFFECT_FLAG_SINGLE_RANGE)
	e4:SetRange(LOCATION_MZONE)
	e4:SetCode(EFFECT_INDESTRUCTABLE_COUNT)
	e4:SetCountLimit(1)
	e4:SetValue(s.valcon)
	c:RegisterEffect(e4)
end
function s.poscon(e,tp,eg,ep,ev,re,r,rp)
	return e:GetHandler():IsAttackPos()
end
function s.posop(e,tp,eg,ep,ev,re,r,rp)
	local c=e:GetHandler()
	if c:IsFaceup() and c:IsRelateToEffect(e) then
		Duel.ChangePosition(c,POS_FACEUP_DEFENSE)
	end
end
function s.valcon(e,re,r,rp)
	return (r&REASON_BATTLE)~=0
end