http://lua.2524044.n2.nabble.com/LightUserData-and-metatables-td3807698.html
"... do you realize that by setting the metatable of a light userdata
you are actually setting the metatable of all light userdata at once ?"
I did not realise this. oops.
设置Full UserData的metatable设置的是一个Full UserData实例的metatable。
设置Light UserData的metatable设置的是所有Light UserData实例的metatable。
Lua的Full UserData、Light UserData和metatable
原文：http://www.cnblogs.com/lilei9110/p/7259148.html