No instances of Tree—why?

models
1

While modeling git objects, I came up with this model:

sig Object {}

sig Tree {
  children: set (Object + Tree)
}

fact {
  no t: Tree | t in t.*children
}

It produces instances, but none with any Tree. Alloy (well, Electrum) says “Predicate is consistent, contrary to expectation.”

What I’m intending to do is say that a tree cannot be its own parent (which may itself be a bad assumption, but I couldn’t make that happen using git hash-object and friends.)

Where have I gone sideways here? It’s been a while since I read Software Abstractions, so a lot of this is trying to get the memory of how to do this back in my fingers.