Reference implementation of built-in subsref, subsasgn and subsindex

I have been trying to understand the intricacies of MATLAB's built-in subscripting. Documentation and the web help by providing many examples (some of them quite obscure), however I haven't been able to find a formal definition of the subscripting rules such that I would understand, once and for all, what is allowed and what is not (and hopefully why).

A second best option would be having access to a reference implementation of the builtin subsref, subasgn and subsindex functions. Then I could trace the interesting cases and deduce the rules.

I understand fully that such code would be awfully inefficient compared to MATLAB's built-ins, however it would be an invaluable learning tool.

Has anybody encountered a reasonably complete reference implementation of the built-in subsref, subsasgn and subsindex functions (preferably from MathWorks)?

Thanks,

Beni