Paul Prescod wrote:
> 
> On 15 Sep 1998 matt@veosystems.com wrote:
> 
> > I know your opinion here.  But inheritance is just a subset of
> > subclass relationships (subclass is an as-a relationship, inheritance
> > is an is-a relationship, and all is-a relationship are also as-a
> > relationships).
> 
> I don't think that there is anything in the word "inheritance" that 
> implies an is-a relationship, though I agree that sometimes it is used 
> that way. 
> 

"Sometimes" meaning rarely, or "sometimes" meaning almost always, but
I'm not ready to concede the point? (I, too, can split hairs!  ;-)
Inheritance and subclassing are both about substitutability.  I think
it would be fair to say that inheritance has _almost always_ been
associated with wholesale borrowing of the structure of the thing
being inherited from, either through copy or reference (through
delegation).  The exceptions, like C++'s private inheritance, are at
the margins.  In what language that you are aware of does inheritance
or subclassing not imply substitutability?

> 
> So let me do so:
> 
> XML element types have three interesting properties: content models,
> attributes and GIs. So to me, "inheritance" between element types would be
> about borrowing some or all of another element types content model,
> attributes or GIs.
> 

If you only borrow some, you lose substitutability.  If you want to
borrow without substitution, just use pe's.  You haven't gained
anything semantically interesting.

> Subclassing, on the other hand, would be about having an element of one 
> type "play the part of" an element of another type, such as a 
> cross-reference "playing" an XLink.
> 

Inheritance means playing the part of the element you are inheriting
from (with a nod towards awkward exceptions at the margin).

Matthew Fuchs
matt@veosystems.com

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