I think it's a little more subtle. A constructor is like a program. In this case, the rules of the language are physics itself. We're familiar with writing sim

I think it's a little more subtle. A constructor is like a program. In this case, the rules of the language are physics itself. We're familiar with writing simple programs in C. We can imagine programs, like the halting problem solver. It turns out that program can't exist, but you kind of have to think about it for a while to understand why. A constructor (as i understand it) is sort of hand rolled assembly (matter) running right on physics. Perhaps there are space unicorns on the dark side of the moon. We can imagine that. It's not really clear if physics actually allows that though. My gut says no, but the rules of the game, much like the rules of the language may allow such a collection of matter to exist. I mean, quines are pretty weird. I wouldn't have thought that was even possible.