Hi Igor,
sorry for bothering you again with this issue but again somebody (squeaksource ;)) removed me from the commiters of the NB project.
Now I have two proposals, a very small one: removing the asm int3, and decrementing the callback counter in #generateCallbackLeaveCodeFor:leaveAddr: so that the vm does not crash on callback error case.
Secondly, I have converged to an implementation of NBExternalEnum (using SharedPool) which I think it is cool, and I wanted to share it with you. Maybe you like it... maybe you don't ... ;)