Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
Level 3 or lower Defense Position Thunder-Type monsters you control cannot be destroyed by battle. Destroy this card if you control a face-up Attack Position monster.
--電磁シールド
--Electromagnetic Shield
local s,id=GetID()
function s.initial_effect(c)
	--activate
	local e1=Effect.CreateEffect(c)
	e1:SetType(EFFECT_TYPE_ACTIVATE)
	e1:SetCode(EVENT_FREE_CHAIN)
	c:RegisterEffect(e1)
	--indestructable
	local e2=Effect.CreateEffect(c)
	e2:SetType(EFFECT_TYPE_FIELD)
	e2:SetCode(EFFECT_INDESTRUCTABLE_BATTLE)
	e2:SetRange(LOCATION_SZONE)
	e2:SetTargetRange(LOCATION_MZONE,0)
	e2:SetTarget(s.infilter)
	e2:SetValue(1)
	c:RegisterEffect(e2)
	--self destroy
	local e3=Effect.CreateEffect(c)
	e3:SetType(EFFECT_TYPE_SINGLE)
	e3:SetProperty(EFFECT_FLAG_SINGLE_RANGE)
	e3:SetRange(LOCATION_SZONE)
	e3:SetCode(EFFECT_SELF_DESTROY)
	e3:SetCondition(s.descon)
	c:RegisterEffect(e3)
end
function s.infilter(e,c)
	return c:IsDefensePos() and c:IsLevelBelow(3) and c:IsRace(RACE_THUNDER)
end
function s.filter(c)
	return c:IsFaceup() and c:IsAttackPos()
end
function s.descon(e)
	return Duel.IsExistingMatchingCard(s.filter,e:GetHandlerPlayer(),LOCATION_MZONE,0,1,nil)
end