As we all know, at the end of "Yesterday's Enterprise" the original timeline is restored and the alternate version where the Enterprise-D is destroyed by Klingons vanishes from existence. But if that's the case, then how did the alternate Tasha Yar still exist on the Enterprise-C when it returned to its own time? Shouldn't she have been erased as well? One explanation I can think of is that when she passed through the temporal distortion, she was somehow shielded from the effects of the continuum restructuring itself, similar to the Enterprise-E going back in time in First Contact. Thoughts, anyone?