allocation :none

[Ken kahn <kahn.pa@Xerox.COM>]

I thought I should explain the original motivation for :allocation
:none.  There are two expected uses of it.

1) A subclass implements an inherited slot procedurally (i.e. it defines
accessor methods) and doesn't need any storage set aside. :allocation
:none just saves some space in each instance but has no semantic import
(except maybe with respect to low-level slot access, i.e. slot-value).

2) A slot is not needed by a subclass.  Declaring it :allocation :none
is both a cheap way to get an error is the instances really do need it
and again a way to save space in instances.

So I guess I'm agreeing with Moon that the slot continues to exist but
certain kinds of access to it are errors.

To some extent, the value of :allocation :none depends upon the style of
object-oriented programming.  One style is to put alot of functionality
in the super-classes which sub-classes turn off.  E.g. an
immovable-window is a subclass of ordinary window and needn't waste
space for slots used only in supporting movement.

----- ken