local Stack = {}
Stack.__index = Stack
function Stack.new() return setmetatable({ items = {} }, Stack) end
function Stack:push(v) table.insert(self.items, v) end
function Stack:pop() return table.remove(self.items) end
function Stack:size() return #self.items end

local s = Stack.new()
s:push("a"); s:push("b")
print(s:size())
print(s:pop())
print(s:pop())
print(s:pop())
print(s:size())