local record Stack
	contents: {any}
	top: number
end
function Stack:push(val: any)
	self.top = self.top + 1
	table.insert(self.contents,val)
end
function Stack:pop(): any
	self.top = self.top -1
	if self.top < 0 then
		error("Stack underflow")
	end
	return table.remove(self.contents)
end
local stack_mt = {__index = Stack}
function Stack:new(): Stack
	return setmetatable({contents = {}, top = 0} as Stack, stack_mt)
end


-- operations
function add(stack: Stack)
	local a: any =stack:pop()
	local b: any=stack:pop()
	if a is number and b is number then
		local c=a+b
		stack:push(c)
	else
		error("invalid operands for add operation!")
	end
end
function dot(stack: Stack)
	print(stack:pop())
end