strange behaviour with mixins before and after class instances

StackOverflowError when mixin is called before instance creations (a and b) in case same mixin on two classes (linked by inheritance)
The script in attachement should be clear enough.
Here is a summary :
B extends A
XCategory is mixin to A and B
a instance of A and b instance of B

When the mixins are applied after the instance creations (as in comments), it works.

The stackoverflow indicates :
java.lang.StackOverflowError
at B.toString(ConsoleScript47:9)
at XCategory.toString(ConsoleScript47:15)
at B.toString(ConsoleScript47:9)
at XCategory.toString(ConsoleScript47:15)

Another point is how to have the following assert code working :

assert a.toString() == "XA"
assert b.toString() == "XBXA" // instead of "XBA"