Containers_pp.ExtExtension node.
Custom nodes can be used to add user-defined behavior to the rendered output. For example, documents might be annotated with ANSI-terminal colors, or with HTML tags.
type 'a t = {pre : Out.t -> 'a -> unit;Printed before the wrapped value.
*)post : Out.t -> 'a -> unit;Printed after the wrapped value.
*)}An extension is a custom document node. It takes a value of type 'a, and a document d, and can output what it wants based on the custom value before and after d is printed.
The extension is considered to have width 0.