Yes, I was having a hunch you are talking about conditioning onto predicates. There is the excellent work by @zennatavares (see: http://proceedings.mlr.press/v97/tavares19a/tavares19a.pdf) that is aiming for this. You can have a look at his package called Omega.jl
We currently don’t have a way to nicely do this in Turing and I’m also not too familiar with those kind of problems. But maybe we could find a way to bridge Omega and Turing at some point. If you can obtain analytical expressions for the Gibbs conditionals then you can at least sample from the conditional in Turing in the very near future (we have a open PR for this atm). But as I said, conditioning on predicates is currently not well established in Turing we didn’t think much about it tbh. But it would definitely be interesting to do.