[Xotcl] Precedence Order

[Scott Gargash]

As I understand it, precedence order is computed as filters then mixins 
the superclasses, and this sort order is applied "globally" to an object. 

In other words:

Class create Base
Class create Derived -superclass Base
Derived d
d info precedence ==> ::Derived ::Base ::xotcl::Object

Class create BaseMixin
Base instmixin add BaseMixin
d info precedence  ==> ::BaseMixin ::Derived ::Base ::xotcl::Object

This behavior seems to violate encapsulation. BaseMixin is intended to 
intercept messages to Base. Derived doesn't know about BaseMixin and 
BaseMixin doesn't know about Derived, yet BaseMixin ends up being the 
first interceptor of messages to Derived.

Since BaseMixin is intended to modify the behavior of Base, it would be 
better BaseMixin preceeded Base (and only Base) in the sort order. 

d info precedence  ==> ::Derived ::BaseMixin ::Base ::xotcl::Object

The current behavior means you can't really add a mixin without 
understanding everywhere that the object being extended is used. Is there 
some benefit to the current behavior that I don't understand?
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