I was lumping clpfd under metaprogramming, since it acts like an extra runtime or dsl and you can't use the prolog debugger to make sense of it. I'll readily admit that prolog+clpfd is very powerful; any goalpost-moving on my part is unintentional.

I found that using peano numbers got a bit further in plain prolog, but it still eventually infloops in at least one of the directions.