diff --git a/bij.ml b/bij.ml
index 6bf1eb4c..3f0bf337 100644
--- a/bij.ml
+++ b/bij.ml
@@ -37,7 +37,11 @@ type _ t =
   | Pair : 'a t * 'b t -> ('a * 'b) t
   | Triple : 'a t * 'b t * 'c t -> ('a * 'b * 'c) t
   | Map : ('a -> 'b) * ('b -> 'a) * 'b t -> 'a t
-  | Switch : ('a -> char) * (char * 'a t) list -> 'a t
+  | Switch : ('a -> char * 'a inject_branch) * (char -> 'a extract_branch) -> 'a t
+and _ inject_branch =
+  | BranchTo : 'b t * 'b * 'a -> 'a inject_branch
+and _ extract_branch =
+  | BranchFrom : 'b t * ('b -> 'a) -> 'a extract_branch
 
 type 'a bij = 'a t
 
@@ -55,7 +59,7 @@ let pair a b = Pair(a,b)
 let triple a b c = Triple (a,b,c)
 
 let map ~inject ~extract b = Map (inject, extract, b)
-let switch select l = Switch (select, l)
+let switch ~inject ~extract = Switch (inject, extract)
 
 exception EOF
 
@@ -238,8 +242,8 @@ module SexpEncode(Sink : SINK) = struct
       | Float, f -> Sink.write_float sink f
       | List bij', l ->
         Sink.write_char sink '(';
-        List.iter
-          (fun x -> Sink.write_char sink ' '; encode bij' x)
+        List.iteri
+          (fun i x -> (if i > 0 then Sink.write_char sink ' '); encode bij' x)
           l;
         Sink.write_char sink ')'
       | Many _, [] -> failwith "Bij.encode: expected non-empty list"
@@ -270,15 +274,11 @@ module SexpEncode(Sink : SINK) = struct
       | Map (inject, _, bij'), x ->
         let y = inject x in
         encode bij' y
-      | Switch (select, l), x ->
-        let c = select x in
-        try
-          let bij' = List.assq c l in
-          encode bij' x
-        with Not_found ->
-          raise (EncodingError "no encoding in switch")
+      | Switch (inject, _), x ->
+        let c, BranchTo (bij', y, _) = inject x in
+        Sink.write_char sink c;
+        encode bij' y
     in encode bij x
-
 end
 
 module SexpDecode(Source : SOURCE) = struct
@@ -347,7 +347,12 @@ module SexpDecode(Source : SOURCE) = struct
       | Map (_, extract, bij') ->
         let x = decode bij' in
         extract x
-      | Switch (_, choices) -> decode_switch choices
+      | Switch (_, extract) ->
+        let c = cur () in
+        let BranchFrom (bij', convert) = extract c in
+        junk ();   (* remove c *)
+        let y = decode bij' in
+        convert y  (* translate back *)
     and decode_open : unit -> unit = fun () -> match cur () with
       | '(' -> junk ()  (* done *)
       | _ -> raise (DecodingError "expected '('")
@@ -377,16 +382,6 @@ module SexpDecode(Source : SOURCE) = struct
       | _ ->
         let x = decode bij in
         decode_list bij (x :: l)
-    and decode_switch : type a. (char * a t) list -> a = fun choices ->
-      let c = cur () in
-      junk ();
-      let bij =
-        try List.assq c choices 
-        with Not_found -> 
-          try List.assq ' ' choices
-          with Not_found -> raise (DecodingError "no choice")
-      in
-      decode bij
     in
     decode bij
 end
diff --git a/bij.mli b/bij.mli
index 14a4cda9..d880985d 100644
--- a/bij.mli
+++ b/bij.mli
@@ -42,7 +42,14 @@ val pair : 'a t -> 'b t -> ('a * 'b) t
 val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t
 
 val map : inject:('a -> 'b) -> extract:('b -> 'a) -> 'b t -> 'a t
-val switch : ('a -> char) -> (char * 'a t) list -> 'a t
+
+type _ inject_branch =
+  | BranchTo : 'b t * 'b * 'a -> 'a inject_branch
+type _ extract_branch =
+  | BranchFrom : 'b t * ('b -> 'a) -> 'a extract_branch
+
+val switch : inject:('a -> char * 'a inject_branch) ->
+             extract:(char -> 'a extract_branch) -> 'a t
   (** discriminates based on the next character. 
       The selection function, with type ['a -> char], is used to select a
       bijection depending on the value.
diff --git a/tests/test_bij.ml b/tests/test_bij.ml
index 791e03db..f7c2d61a 100644
--- a/tests/test_bij.ml
+++ b/tests/test_bij.ml
@@ -26,6 +26,32 @@ let test_intlist n () =
   let l' = SexpStr.of_string ~bij s in
   OUnit.assert_equal ~printer:pp_int_list l l'
 
+type term =
+  | Const of string
+  | Int of int
+  | App of term list
+
+let bij_term =
+  let rec mk_bij () =
+    switch
+    ~inject:(fun t -> match t with
+      | Const s -> 'c', BranchTo (string_, s, t)
+      | Int i -> 'i', BranchTo (int_, i, t)
+      | App l -> 'a', BranchTo (list_ (mk_bij ()), l, t))
+    ~extract:(function
+      | 'c' -> BranchFrom (string_, fun x -> Const x)
+      | 'i' -> BranchFrom (int_, fun x -> Int x)
+      | 'a' -> BranchFrom (list_ (mk_bij ()), fun l -> App l)
+      | _ -> raise (DecodingError "unexpected case switch"))
+  in mk_bij ()
+
+let test_rec () =
+  let t = App [Const "foo"; App [Const "bar"; Int 1; Int 2]; Int 3; Const "hello"] in
+  let s = SexpStr.to_string ~bij:bij_term t in
+  Printf.printf "to: %s\n" s;
+  let t' = SexpStr.of_string ~bij:bij_term s in
+  OUnit.assert_equal t t'
+
 let suite =
   "test_bij" >:::
     [ "test_int2" >:: test_int2;
@@ -33,4 +59,5 @@ let suite =
       "test_intlist10" >:: test_intlist 10;
       "test_intlist100" >:: test_intlist 100;
       "test_intlist10_000" >:: test_intlist 10_000;
+      "test_rec" >:: test_rec;
     ]