function NewStack(t)
local Stack = {
size_of = function(self)
	local size_of=#self
	return size_of
end,
push = function(self, ...)
for _, v in ipairs{...} do
	self[#self+1] = v
	end
end,
pop = function(self, num)
	local num = num or 1
	if num > #self then
	return nil
	--error("underflow in NewStack-created stack")
	end
local ret = {}
	for i = num, 1, -1 do
	ret[#ret+1] = table.remove(self)
	end
return unpack(ret)
end
}
return setmetatable(t or {}, {__index = Stack})
end