[DGD] private inheritance

[Felix A. Croes]

Private inheritance means that the functions and variables in the
inherited object will be known only in the object that does the
inheriting.  If the latter object itself is inherited, the functions
and variables will be hidden.  Effectively, the functions and
variables behave as if they are private in the object that inherits
them.

If an object is both privately and non-privately inherited, the
non-private instance will replace the private one.

A collision between two non-privately inherited functions cannot
be masked by a function that is privately inherited.  For example,
if A and B define the function foo, and C inherits A and B and
redefines foo, then object D which inherits A and B normally and
C privately will not compile unless it also redefines foo to mask
the collision from A and B, even though C does the same thing.

It is not possible to inherit an object with a nomask function
privately.  However, if A has a nomask function and B inherits A,
C can inherit B privately if it first inherits A normally.

Implementing private inheritance involved comprehensive changes
to DGD's inheritance implementation, one of the most complex
parts of the server.  Consider version 1.1.47 experimental and
subject to change.

Regards,
Dworkin