Lets call them built-in types: Object, String, Number, Boolean and Date. In the mse file, you must not include them and can refer to them using named references, eg (ref: String). An implementation of named references must implement the lookup of names such that these built-in types precede any user defined name, ie it must be impossible to shadow these names.
Will you make sure that I will be able to load a model of the class String in Moose via mse. I guess so but I decided to ask.