Minimal Class Implementation

function class(obj)
  obj = obj or {}
  obj.init = obj.init or function() end

  function obj:new(...)
    local instance = setmetatable({__class = obj}, {__index = obj})
    return instance, instance:init(...)
  end

  function obj:extend(t)
    t = t or {}
    for k,v in pairs(obj) do
      if not t[k] then
        t[k] = v
      end
    end
    return class(t)
  end

  return setmetatable(obj, {__call = obj.new})
end