Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
Once per turn: You can change this card to face-down Defense Position. If this card is flipped face-up: You can destroy monsters on the field, up to the number of other "Geargia" monsters you currently control.
--ギアギアンカー
--Geargianchor
local s,id=GetID()
function s.initial_effect(c)
	--turn set
	local e1=Effect.CreateEffect(c)
	e1:SetCategory(CATEGORY_POSITION)
	e1:SetType(EFFECT_TYPE_IGNITION)
	e1:SetRange(LOCATION_MZONE)
	e1:SetTarget(s.postg)
	e1:SetOperation(s.posop)
	c:RegisterEffect(e1)
	--destroy
	local e2=Effect.CreateEffect(c)
	e2:SetCategory(CATEGORY_DESTROY)
	e2:SetType(EFFECT_TYPE_SINGLE+EFFECT_TYPE_TRIGGER_O)
	e2:SetCode(EVENT_FLIP)
	e2:SetProperty(EFFECT_FLAG_DELAY+EFFECT_FLAG_DAMAGE_STEP)
	e2:SetCondition(s.descon)
	e2:SetTarget(s.destg)
	e2:SetOperation(s.desop)
	c:RegisterEffect(e2)
end
s.listed_series={SET_GEARGIA}
function s.postg(e,tp,eg,ep,ev,re,r,rp,chk)
	local c=e:GetHandler()
	if chk==0 then return c:IsCanTurnSet() and c:GetFlagEffect(id)==0 end
	c:RegisterFlagEffect(id,RESET_EVENT|(RESETS_STANDARD_PHASE_END&~RESET_TURN_SET),0,1)
	Duel.SetOperationInfo(0,CATEGORY_POSITION,c,1,0,0)
end
function s.posop(e,tp,eg,ep,ev,re,r,rp)
	local c=e:GetHandler()
	if c:IsRelateToEffect(e) and c:IsFaceup() then
		Duel.ChangePosition(c,POS_FACEDOWN_DEFENSE)
	end
end
function s.descon(e,tp,eg,ep,ev,re,r,rp)
	return Duel.IsExistingMatchingCard(aux.FaceupFilter(Card.IsSetCard,SET_GEARGIA),tp,LOCATION_MZONE,0,1,e:GetHandler())
end
function s.destg(e,tp,eg,ep,ev,re,r,rp,chk)
	if chk==0 then return Duel.IsExistingMatchingCard(aux.TRUE,tp,LOCATION_MZONE,LOCATION_MZONE,1,nil) end
	local g=Duel.GetMatchingGroup(aux.TRUE,tp,LOCATION_MZONE,LOCATION_MZONE,nil)
	Duel.SetOperationInfo(0,CATEGORY_DESTROY,g,1,0,0)
end
function s.desop(e,tp,eg,ep,ev,re,r,rp)
	local ct=Duel.GetMatchingGroupCount(aux.FaceupFilter(Card.IsSetCard,SET_GEARGIA),tp,LOCATION_MZONE,0,e:GetHandler())
	if ct==0 then return end
	Duel.Hint(HINT_SELECTMSG,tp,HINTMSG_DESTROY)
	local g=Duel.SelectMatchingCard(tp,aux.TRUE,tp,LOCATION_MZONE,LOCATION_MZONE,1,ct,nil)
	if #g>0 then
		Duel.HintSelection(g)
		Duel.Destroy(g,REASON_EFFECT)
	end
end