Datatype.Undefined
Each values in these modules are undefined. The usual way to use it is: module X: Datatype.S = struct
include Undefined
type t = ...
let reprs = ...
let name = ...
let mem_project = ... (* Usually, Datatype.never_any_project *)
(* define only useful functions for this datatype *)
end
val structural_descr : Structural_descr.t
val mem_project : (Project_skeleton.t -> bool) -> 'a -> bool