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diff --git a/lib/explore.ml b/lib/explore.ml
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+++ b/lib/explore.ml
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+
+(*i $Id$ i*)
+
+open Format
+
+(*s Definition of a search problem. *)
+
+module type SearchProblem = sig
+  type state
+  val branching : state -> state list
+  val success : state -> bool
+  val pp : state -> unit
+end
+
+module Make = functor(S : SearchProblem) -> struct
+
+  type position = int list
+
+  let rec pp_position = function
+    | [] -> ()
+    | [i] -> printf "%d" i
+    | i :: l -> pp_position l; printf ".%d" i
+
+  (*s Depth first search. *)
+
+  let rec depth_first s = 
+    if S.success s then s else depth_first_many (S.branching s)
+  and depth_first_many = function
+    | [] -> raise Not_found
+    | s :: l -> try depth_first s with Not_found -> depth_first_many l
+
+  let debug_depth_first s = 
+    let rec explore p s =
+      pp_position p; S.pp s;
+      if S.success s then s else explore_many 1 p (S.branching s)
+    and explore_many i p = function
+      | [] -> 
+	  raise Not_found
+      | s :: l -> 
+	  try explore (i::p) s with Not_found -> explore_many (succ i) p l
+    in
+    explore [1] s
+
+  (*s Breadth first search. We use functional FIFOS à la Okasaki. *)
+
+  type 'a queue = 'a list * 'a list
+
+  exception Empty
+
+  let empty = [],[]
+
+  let push x (h,t) = (x::h,t)
+
+  let pop = function
+    | h, x::t -> x, (h,t)
+    | h, [] -> match List.rev h with x::t -> x, ([],t) | [] -> raise Empty
+
+  let breadth_first s = 
+    let rec explore q =
+      try 
+	let (s, q') = pop q in enqueue q' (S.branching s)
+      with Empty ->
+	raise Not_found
+    and enqueue q = function
+      | [] -> explore q
+      | s :: l -> if S.success s then s else enqueue (push s q) l
+    in
+    enqueue empty [s]
+
+  let debug_breadth_first s = 
+    let rec explore q =
+      try 
+	let ((p,s), q') = pop q in 
+	enqueue 1 p q' (S.branching s)
+      with Empty ->
+	raise Not_found
+    and enqueue i p q = function
+      | [] -> 
+	  explore q
+      | s :: l ->
+	  let ps = i::p in 
+	  pp_position ps; S.pp s;
+	  if S.success s then s else enqueue (succ i) p (push (ps,s) q) l
+    in
+    enqueue 1 [] empty [s]
+
+end