[Oberon] Type compatibility rules for Pointers

[Tomas Kral]

> Section 5.2 Compatibility of a Base Type and Its Extension, in
> Hanspeter Mössenböck's book: Object-Oriented Programming in Oberon-2.

Hi Chris, 

Yes thank you for the pointer, been there, done that, have some
understanding.

Type extension, and procedure variables, are the basis of OOP.
I learn to use, though often surprised things are possible, unlike in
Modula-2.

Seeing some ETH Oberon modules, COPY() is quite frequently used,
where in Oberon-07, just `:=' assignment will do.

-- 
Tomas Kral <thomas.kral at email.cz>