I removed the useless module Item.

Item.mli

@@ -1,6 +0,0 @@
-module type S =
-  sig
-    type t
-    val compare   : t -> t -> int
-    val to_string : t -> string
-  end

Partition.mli

@@ -1,11 +1,11 @@
 (** This module offers the abstract data type of a partition of
     classes of equivalent items (Union & Find). *)
 
-(** The items are of type [PrintOrdType.t], that is, they have to obey
+(** The items are of type [Item.t], that is, they have to obey
     a total order, but also they must be printable to ease
-    debugging. The signature [PrintOrdType] is the input signature of
+    debugging. The signature [Item] is the input signature of
     the functor {!Partition.Make}. *)
-module type PrintOrdType =
+module type Item =
   sig
     (** Type of items *)
     type t
@@ -61,4 +61,4 @@ module type S =
     val is_equiv : item -> item -> partition -> bool
   end
 
-module Make (Ord : PrintOrdType) : S with type item = Ord.t
+module Make (Ord : Item) : S with type item = Ord.t

Partition0.ml

@@ -1,6 +1,6 @@
 (* Naive persistent implementation of Union/Find: O(n^2) worst case *)
 
-module Make (Item: Item.S) =
+module Make (Item: Partition.Item) =
   struct
 
     type item = Item.t

Partition1.ml

@@ -6,7 +6,7 @@
    height of [i] (_not_ [j]).
 *)
 
-module Make (Item: Item.S) =
+module Make (Item: Partition.Item) =
   struct
 
     type item = Item.t

Partition2.ml

@@ -1,7 +1,7 @@
 (** Persistent implementation of the Union/Find algorithm with
     height-balanced forests and without path compression. *)
 
-module Make (Item: Item.S) =
+module Make (Item: Partition.Item) =
   struct
 
     type item = Item.t

Partition3.ml

@@ -1,7 +1,7 @@
 (* Destructive implementation of union/find with height-balanced
    forests but without path compression: O(n*log(n)). *)
 
-module Make (Item: Item.S) =
+module Make (Item: Partition.Item) =
   struct
 
     type item = Item.t

build.sh

@@ -1,6 +1,5 @@
 #!/bin/sh
 set -x
-ocamlfind ocamlc -strict-sequence -w +A-48-4 -c Item.mli
 ocamlfind ocamlc -strict-sequence -w +A-48-4 -c Partition.mli
 ocamlfind ocamlopt -strict-sequence -w +A-48-4 -c Partition0.ml
 ocamlfind ocamlopt -strict-sequence -w +A-48-4 -c Partition2.ml