I was wondering about that. I think it is about having the ability to go A->B without being able to go B->A. But, as with many of the Pearls, it is heavily con

I was wondering about that. I think it is about having the ability to go A->B without being able to go B->A. But, as with many of the Pearls, it is heavily context dependent. And if you aren't well versed in "primitive" computing paradigms, you'll be confused.