I tried implementing between/3: https://github.com/guregu/predicates/blob/master/between.go
Would you be interested in adding it here? I imagine this could be implemented in Prolog instead of natively pretty easily 😇.

I wanted to make it lazily create the choice points so I tried using engine.Repeat, but I'm not sure if I did it right.
I also added a check for overflowing but I'm not sure if we want that.

P.S. @ichiban, thank you very much for this project. I am having a lot of fun using it. It has led me down the Prolog rabbit-hole and changed my perspective on programming.