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+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2016     *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+(* Use case:
+     You have a data structure that needs to be marshalled but it contains
+     unmarshallable data (like a closure, or a file descriptor).  Actually
+     you don't need this data to be preserved by marshalling, it just happens
+     to be there.
+     You could produced a trimmed down data structure, but then, once
+     unmarshalled, you can't used the very same code to process it, unless you
+     re-inject the trimmed down data structure into the standard one, using
+     dummy values for the unmarshallable stuff.
+     Similarly you could change your data structure turning all types [bad]
+     into [bad option], then just before marshalling you set all values of type
+     [bad option] to [None].  Still this pruning may be expensive and you have
+     to code it.
+
+   Desiderata:
+     The marshalling operation automatically discards values that cannot be
+     marshalled or cannot be properly unmarshalled.
+
+   Proposed solution:
+     Turn all occurrences of [bad] into [bad key] in your data structure.
+     Use [crate bad_val] to obtain a unique key [k] for [bad_val], and store
+     [k] in the data structure.  Use [get k] to obtain [bad_val].
+
+     An ['a key] can always be marshalled.  When marshalled, a key loses its
+     value.  The function [get] raises Not_found on unmarshalled keys.
+     
+     If a key is garbage collected, the corresponding value is garbage
+     collected too (unless extra references to it exist).
+     In short no memory management hassle, keys can just replace their
+     corresponding value in the data structure.  *)
+
+type 'a key
+
+val create : 'a -> 'a key
+
+(* May raise InvalidKey *)
+exception InvalidKey
+val get : 'a key -> 'a
+
+(* These never fail. *)
+val iter_opt : 'a key -> ('a -> unit) -> unit
+val default : 'a key -> 'a -> 'a
+
+val clear : unit -> unit