A question about Self encapsulation

[takuro ikejiri]

Hi all,

I am a university student and interested in prototype-base  language.
I have a question:

http://research.sun.com/self/release_4.0/Self-4.0/Tutorial/Language/FinishingTouches/FinishingTouches.html

according to this page,
"The distinction between public and private is purely for documentation.
Earlier versions of Self enforced privacy (allowing only sends to self
to locate private slots) but this scheme was found to be unworkable."

My question is "what is unworkable about Self encapsulation ?".

I also read  a paper ( parents are shared parts of objects:
Inheritance and encapsulation in SELF,1991),
and learned about inheritance-based encapsulation.
I think this approach is not good because an encapsulated module
cannot prohibit access from outside
(because an object gain access to the module by becoming a child of
the module).

I'd appreciate it if you could answer this question.
Thanks in advance.