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?