diff --git a/src/let-syntax/dune b/src/let-syntax/dune
new file mode 100644
index 00000000..0d7b1143
--- /dev/null
+++ b/src/let-syntax/dune
@@ -0,0 +1,2 @@
+(executable (name pp))
+(ocamllex pp)
diff --git a/src/let-syntax/pp.mll b/src/let-syntax/pp.mll
new file mode 100644
index 00000000..560460b2
--- /dev/null
+++ b/src/let-syntax/pp.mll
@@ -0,0 +1,186 @@
+{
+open StdLabels
+open Printf
+
+let fname = Sys.argv.(1)
+let fname_gen = fname ^ ".generated"
+
+let lexeme_len lb =
+  Lexing.lexeme_end lb - Lexing.lexeme_start lb
+
+let fail lb msg =
+  let start = Lexing.lexeme_start_p lb in
+  let stop = Lexing.lexeme_end_p lb in
+  Printf.eprintf
+    "File %S, line %d, characters %d-%d:\n\
+     Error: %s\n%!"
+    fname start.pos_lnum (start.pos_cnum - start.pos_bol)
+    (stop.pos_cnum - start.pos_bol)
+    msg;
+  exit 1
+
+(* Line number in generated file *)
+let lnum = ref 1
+
+let ps s =
+  String.iter s ~f:(function
+    | '\n' -> incr lnum
+    | _    -> ());
+  print_string s
+let pc = function
+  | '\n' -> incr lnum; print_char '\n'
+  | c    -> print_char c
+let npc n c = for _ = 1 to n do pc c done
+let pf fmt = ksprintf ps fmt
+
+let gen_id =
+  let n = ref 0 in
+  fun () ->
+    incr n;
+    sprintf "__x%d__" !n
+
+let buf = Buffer.create 512
+let add_lexeme lb = Buffer.add_string buf (Lexing.lexeme lb)
+
+type mode = Generated_code | Source_code
+
+let mode = ref Generated_code
+
+let enter_generated_code () =
+  mode := Generated_code;
+  pc '\n';
+  pf "# %d %S\n" !lnum fname_gen
+
+let enter_source_code (pos : Lexing.position) =
+  mode := Source_code;
+  pc '\n';
+  pf "# %d %S\n" pos.pos_lnum fname;
+  let col = pos.pos_cnum - pos.pos_bol in
+  npc col ' '
+
+let pass_through lb =
+  if !mode = Generated_code then
+    enter_source_code (Lexing.lexeme_start_p lb);
+  ps (Lexing.lexeme lb)
+
+type and_or_in = And | In
+}
+
+let space = [' ' '\t']
+let newline = '\n' | "\r\n"
+let id =  ['a'-'z' '_'] ['A'-'Z' 'a'-'z' '_' '\'' '0'-'9']*
+
+rule main col = parse
+  | eof
+    { In }
+  | newline
+    { pass_through lexbuf;
+      Lexing.new_line lexbuf;
+      after_newline col lexbuf
+    }
+  | " in" newline
+    { pass_through lexbuf;
+      Lexing.new_line lexbuf;
+      after_in_and_newline col lexbuf
+    }
+  | _
+    { pass_through lexbuf;
+      main col lexbuf
+    }
+  | id as id
+    { pass_through lexbuf;
+      match id with
+      | "let" -> after_let col (Lexing.lexeme_start_p lexbuf) lexbuf
+      | _     -> main col lexbuf
+    }
+
+and after_let col pos = parse
+  | "%" (id as id)
+    { if id <> "map" then begin
+        pass_through lexbuf;
+        main col lexbuf
+      end else begin
+        let col' = pos.Lexing.pos_cnum - pos.Lexing.pos_bol in
+        let rec loop () =
+          let pos = Lexing.lexeme_end_p lexbuf in
+          enter_generated_code ();
+          let id = gen_id () in
+          ps id;
+          lhs lexbuf;
+          let pattern = Buffer.contents buf in
+          Buffer.clear buf;
+          (id, (pos, pattern)) ::
+          match main col' lexbuf with
+          | And -> loop ()
+          | In  -> []
+        in
+        let ids, patterns = List.split (loop ()) in
+        enter_generated_code ();
+        ps "Let_syntax.(fun f -> const f";
+        List.iter ids ~f:(pf "$ %s");
+        ps ") @@ fun ";
+        List.iter patterns ~f:(fun (pos, pattern) ->
+          pc '(';
+          enter_source_code pos;
+          ps pattern;
+          pc ')');
+        ps "->";
+        mode := Generated_code;
+        main col lexbuf
+      end
+    }
+  | ""
+    { main col lexbuf }
+
+and after_newline col = parse
+  | space*
+    { pass_through lexbuf;
+      if lexeme_len lexbuf = col then
+        after_indent col lexbuf
+      else
+        main col lexbuf
+    }
+
+and after_indent col = parse
+  | id as id
+    { pass_through lexbuf;
+      match id with
+      | "and" -> And
+      | "in"  -> In
+      | _     -> fail lexbuf "'and' or 'in' keyword expected"
+    }
+
+and after_in_and_newline col = parse
+  | space* newline
+    { pass_through lexbuf;
+      Lexing.new_line lexbuf;
+      after_in_and_newline col lexbuf
+    }
+  | space*
+    { pass_through lexbuf;
+      if lexeme_len lexbuf = col then
+        In
+      else
+        main col lexbuf
+    }
+
+and lhs = parse
+  | eof
+    { ()
+    }
+  | newline as s
+    { Buffer.add_string buf s;
+      Lexing.new_line lexbuf;
+      lhs lexbuf
+    }
+  | "="
+    { pass_through lexbuf
+    }
+  | _ as c
+    { Buffer.add_char buf c;
+      lhs lexbuf
+    }
+
+{
+  let (And | In) = main (-1) (Lexing.from_channel (open_in_bin fname))
+}