Out of curiosity, how did you encounter this paper ciderpunx? The authors aren't familiar to me from logic/computing, but perhaps they're combinatorialists. If

Out of curiosity, how did you encounter this paper ciderpunx? The authors aren't familiar to me from logic/computing, but perhaps they're combinatorialists. If I find time I'll look at the paper in more detail as it's a question that I pondered but never bothered to answer myself when I studied the λ calculus.