local function iter(lst, elem)
    if not elem then elem = lst.first
    else elem = elem.next end
    return elem
end

local list_mt = {}
list_mt.__index = list_mt
list_mt.push_back = function(self, val) 
    if self.last == nil then
        self.first = {value = val, prev = nil, next = nil}
        self.last = self.first
    else
        self.last.next = {value = val, prev = self.last, next = nil}
        self.last = self.last.next
    end
end

list_mt.remove = function(self, elem)     
    if elem.next then elem.next.prev = elem.prev end
    if elem.prev then elem.prev.next = elem.next end
    if self.last == elem then self.last = elem.prev end
    if self.first == elem then self.first = elem.next end
    elem = nil
end

-- Iterator for List
function ilist(lst)
    return iter, lst, nil
end

-- Create new List
function NewList()
    local self = {
        first = nil,
        last = nil
    }
    setmetatable(self, list_mt)
    return self
end

lst = NewList()
lst:push_back(123)
lst:push_back(456)
lst:push_back(789)    
lst:push_back(456)
lst:push_back(789)

local cnt = 0
for elem in ilist(lst) do
    if elem.value == 456 then
        lst:remove(elem)
    end
    cnt = cnt + 1
    print(cnt, elem.value)        
end

print("-----")

cnt = 0
for elem in ilist(lst) do
    cnt = cnt + 1
    print(cnt, elem.value)                
end