First cut at spellcheck filter

tools/prepare_spellcheck.ml

+let read_line () = 
+  try
+    Some (read_line ())
+  with End_of_file -> None
+
+let strstr' p s n =
+  try
+    Some (Str.search_forward (Str.regexp_string p) s n)
+  with Not_found -> None
+
+let strstr p s = strstr' p s 0
+
+let prefix s1 s2 = String.length s2 >= String.length s1 && Str.string_before s2 (String.length s1) = s1
+
+let sanitize s =
+  let rec san n acc =
+    try
+      let pos = Str.search_forward (Str.regexp "\\[\\|%\\|#") s n in
+      let ender = match s.[pos] with
+      | '[' -> ']'
+      | _ -> s.[pos] in
+      let pos' = String.index_from s (pos+1) ender in
+      san (pos'+1) (acc ^ String.sub s n (pos-n))
+    with Not_found -> acc ^ Str.string_after s n
+  in san 0 ""
+
+let rec initial () =
+  match read_line () with
+  | None -> ()
+  | Some line ->
+      match strstr "(**" line with
+      | None -> initial ()
+      | Some pos ->
+	  match strstr "*)" line with
+	  | None ->
+	      begin match strstr "[[" line with
+	      | None ->
+		  print_endline (sanitize (Str.string_after line (pos+3)));
+		  comment ()
+	      | Some _ -> runTo "]]"
+	      end
+	  | Some pos' ->
+	      let rest = Str.string_after line (pos+3) in
+	      if not (prefix " printing" rest || prefix " begin" rest || prefix " end" rest) then
+		print_endline (sanitize (String.sub line (pos+3) (pos' - (pos+3))));
+	      initial ()
+
+and comment () =
+  match read_line () with
+  | None -> ()
+  | Some line ->
+      match strstr "*)" line with
+      | None ->
+	  begin match strstr "[[" line with
+	  | None ->
+	      begin match strstr "<<" line with
+	      | None ->
+		  print_endline (sanitize line);
+		  comment ()
+	      | Some _ -> runTo ">>"
+	      end
+	  | Some _ -> runTo "]]"
+	  end
+      | Some pos ->
+	  print_endline (sanitize (Str.string_before line pos));
+	  initial ()
+
+and runTo ender =
+  match read_line () with
+  | None -> ()
+  | Some line ->
+      match strstr ender line with
+      | None -> runTo ender
+      | _ ->
+	  match strstr "*)" line with
+	  | None -> comment ()
+	  | _ -> initial ()
+
+let () = initial ()