Function registries 

pub fn registries() -> &'static Registries

Every registry in the process, assembled the first time somebody asks.

Built once and immutable after. A registry that could gain an entry after a query has planned against it is a registry that makes two runs of the same query incomparable, and comparing runs is the entire point of having one.

Public because EXPLAIN prints the seam section out of it and EXPLAIN is rendered above this crate, in rudb. The optimizer cannot reach it, being under this crate in the layer rule, so the caller that has both hands it over.