Class = {
  classname = "Class aka Object aka Root-Of-Tree",
  new = function(s,t)
    s.__index = s
    local instance = setmetatable(t or {}, s)
    instance.parent = s
    return instance
  end
}

Animal = Class:new{classname="Animal", speak=function(s) return s.voice or "("..s.classname.." has no voice)" end }
Cat = Animal:new{classname="Cat", voice="meow"}
Dog = Animal:new{classname="Dog", voice="woof"}
Lab = Dog:new{classname="Lab", voice="bark"}
Collie = Dog:new{classname="Collie"}