Module `Relude_HMap.Witness`

Witness contains a type t with an existential type variable - a type that we capture, but lose knowledge of once captured.

t(_) is an extensible variant type, indicated by the ... This is used below in WITNESS so that we can add new cases to the Witness.t(_) type on demand, so that we can create witnesses for any new types we encounter going forward.

type t(_) = ..;