Mermail
When this card destroys an opponent's monster by battle and sends it to the Graveyard: You can discard 1 WATER monster to the Graveyard; Special Summon 1 "Mermail" monster from your Deck in face-up Defense Position. You can only use the effect of "Mermail Abyssnose" once per turn.
local SETCARD_ID_0 = 0x74 --Mermail

local SETCARD_ID_0 = 0x74 --Mermail
local s, id, o = GetID()
function s.initial_effect(c)
	--special summon
	local e1 = Effect.CreateEffect(c)
	e1:SetDescription(aux.Stringid(id, 0))
	e1:SetCategory(CATEGORY_SPECIAL_SUMMON)
	e1:SetCode(EVENT_BATTLE_DESTROYING)
	e1:SetType(EFFECT_TYPE_SINGLE + EFFECT_TYPE_TRIGGER_O)
	e1:SetCountLimit(1, id)
	e1:SetCondition(aux.bdogcon)
	e1:SetCost(s.spcost)
	e1:SetTarget(s.sptg)
	e1:SetOperation(s.spop)
	c:RegisterEffect(e1)
end
function s.cfilter(c)
	return c:IsAttribute(ATTRIBUTE_WATER) and c:IsDiscardable() and c:IsAbleToGraveAsCost()
end
function s.spcost(e, tp, eg, ep, ev, re, r, rp, chk)
	if chk == 0 then
		return Duel.IsExistingMatchingCard(s.cfilter, tp, LOCATION_HAND, 0, 1, nil)
	end
	Duel.DiscardHand(tp, s.cfilter, 1, 1, REASON_COST + REASON_DISCARD)
end
function s.filter(c, e, tp)
	return c:IsSetCard(SETCARD_ID_0) and c:IsCanBeSpecialSummoned(e, 0, tp, false, false, POS_FACEUP_DEFENSE)
end
function s.sptg(e, tp, eg, ep, ev, re, r, rp, chk)
	if chk == 0 then
		return Duel.GetLocationCount(tp, LOCATION_MZONE) > 0 and Duel.IsExistingMatchingCard(s.filter, tp, LOCATION_DECK, 0, 1, nil, e, tp)
	end
	Duel.SetOperationInfo(0, CATEGORY_SPECIAL_SUMMON, nil, 1, tp, LOCATION_DECK)
end
function s.spop(e, tp, eg, ep, ev, re, r, rp)
	if Duel.GetLocationCount(tp, LOCATION_MZONE) <= 0 then
		return
	end
	Duel.Hint(HINT_SELECTMSG, tp, HINTMSG_SPSUMMON)
	local g = Duel.SelectMatchingCard(tp, s.filter, tp, LOCATION_DECK, 0, 1, 1, nil, e, tp)
	if g:GetCount() > 0 then
		Duel.SpecialSummon(g, 0, tp, tp, false, false, POS_FACEUP_DEFENSE)
	end
end


Mermail
When this card destroys an opponent's monster by battle and sends it to the Graveyard: You can discard 1 WATER monster to the Graveyard; Special Summon 1 "Mermail" monster from your Deck in face-up Defense Position. You can only use the effect of "Mermail Abyssnose" once per turn.
-- Mermail
local SETCARD_ID_0 = 0x74 --Mermail
local s, id, o = GetID()
function s.initial_effect(c)
	--special summon
	local e1 = Effect.CreateEffect(c)
	e1:SetDescription(aux.Stringid(id, 0))
	e1:SetCategory(CATEGORY_SPECIAL_SUMMON)
	e1:SetCode(EVENT_BATTLE_DESTROYING)
	e1:SetType(EFFECT_TYPE_SINGLE + EFFECT_TYPE_TRIGGER_O)
	e1:SetCountLimit(1, id)
	e1:SetCondition(aux.bdogcon)
	e1:SetCost(s.spcost)
	e1:SetTarget(s.sptg)
	e1:SetOperation(s.spop)
	c:RegisterEffect(e1)
end
function s.cfilter(c)
	return c:IsAttribute(ATTRIBUTE_WATER) and c:IsDiscardable() and c:IsAbleToGraveAsCost()
end
function s.spcost(e, tp, eg, ep, ev, re, r, rp, chk)
	if chk == 0 then
		return Duel.IsExistingMatchingCard(s.cfilter, tp, LOCATION_HAND, 0, 1, nil)
	end
	Duel.DiscardHand(tp, s.cfilter, 1, 1, REASON_COST + REASON_DISCARD)
end
function s.filter(c, e, tp)
	return c:IsSetCard(SETCARD_ID_0) and c:IsCanBeSpecialSummoned(e, 0, tp, false, false, POS_FACEUP_DEFENSE)
end
function s.sptg(e, tp, eg, ep, ev, re, r, rp, chk)
	if chk == 0 then
		return Duel.GetLocationCount(tp, LOCATION_MZONE) > 0 and Duel.IsExistingMatchingCard(s.filter, tp, LOCATION_DECK, 0, 1, nil, e, tp)
	end
	Duel.SetOperationInfo(0, CATEGORY_SPECIAL_SUMMON, nil, 1, tp, LOCATION_DECK)
end
function s.spop(e, tp, eg, ep, ev, re, r, rp)
	if Duel.GetLocationCount(tp, LOCATION_MZONE) <= 0 then
		return
	end
	Duel.Hint(HINT_SELECTMSG, tp, HINTMSG_SPSUMMON)
	local g = Duel.SelectMatchingCard(tp, s.filter, tp, LOCATION_DECK, 0, 1, 1, nil, e, tp)
	if g:GetCount() > 0 then
		Duel.SpecialSummon(g, 0, tp, tp, false, false, POS_FACEUP_DEFENSE)
	end
end

