The Edn type given here is closed. That's a correct definition of Edn, which is a closed sum. Edn accomplishes extensibility via the Tag type. However, not all Clojure data is Edn. In order to implement the clmap and clget functions with their full generality, they need to support an open set of types. For example, both `#inst "..."` and `(eval '(Date. ...))` are separate types: TaggedLiteral and java.util.Date respectively.

You need either Dynamic or existentials because Clojure enables you to pass data structures between two functions expecting collection elements of differing capabilities without either A) whole program / inter-module analysis or B) an O(N) type translation.