Lua Stack
-- OOP boilerplate - create a class, add __index, then make a constructor
Stack = {}
Stack.__index = Stack
function Stack.new() return setmetatable({}, Stack) end
-- put a new object onto a stack
function Stack:push(input)
self[#self+1] = input
end
-- take an object off a stack
function Stack:pop()
assert(#self > 0, "Stack underflow")
local output = self[#self]
self[#self] = nil
return output
end