GTy.Builderval dyn : unit -> Ty.tdyn () returns a fresh instance of the dyn type for the builder. Each occurrence of dyn in a type should use a fresh instance.
refresh ty refreshes the instances of dyn in ty. This function should be called when combining multiple occurrences of a gradual type that is in the process of being built.
val non_gradual : Ty.t -> boolnon_gradual ty returns true if and only if ty is not gradual, i.e. it has no instance of dyn.