Exactly. It's like with vampires. Back when I writing the UNDERWORLD books, I was surprised to discover how many people seemed to think there was a universal rulebook for vampires that every book or movie or TV show needed to follow: "This is stupid. Everyone knows vampires can't have babies! BUFFY said so!" I kept having to explain that UNDERWORLD vampires did not necessarily follow the same rules as BUFFY vampires . . . or TRUE BLOOD vampires . . . or Anne Rice vampires . . . or Bram Stoker vampires or whatever. Same with time-travel. Certainly, you don't want to rewrite the rules mid-story, but there's no reason that every time-travel story, from H.G. Wells on, has to follow the same playbook. Or even that every method of time-travel operations under the same conditions. Clearly, the atavachron operates differently than, say, the Guardian of Forever . . . ..