ocamllex syntax error

I get the following laconic error message :

$ ocamllex huong.ml
File "huong.mll", line 2, character 2: syntax error.

Here is my complete file :

{

  let create_hashtable size init =
    let tbl = Hashtbl.create size in
    List.iter (fun (key, data) -> Hashtbl.add tbl key data) init;
    tbl

  type token =
    | OP of char
    | INT of int
    | FLOAT of float
    | CHAR of char
    | PUNCTUATION_MARK of char
    | RESERVED of string
    | UNRESERVED of string
    
  let reserved_words=
     [
       "AUTO_INCREMENT";
       "CHARSET";
       "COLLATE";
       "CREATE";
       "DEFAULT";
       "ENGINE";
       "INSERT";
       "INTO";
       "KEY";
       "NOT";
       "NULL";
       "PRIMARY";
       "SET";
       "SQL_MODE";
       "time_zone";
       "VALUES"
     ]  
     
  let punctuation_marks=
     [
       ';';
       '\"';
       '\'';
       '=';
       '`';
       ',';
       '?';
       ':';
       '(';
       ')';
       '{';
       '}';   
     ]   
    
  let keyword_list =  
   (List.map (fun c->(String.make 1 c,PUNCTUATION_MARK c)) reserved_words )
  @(List.map (fun x->(x,RESERVED x)) reserved_words )

  let keyword_table = 
    create_hashtable (List.length keyword_list) keyword_list
    
  let accu=ref([])
  
  let memorize x=(accu:=x::(!accu))  

}

let digit = ['0'-'9']
let id = ['a'-'z' 'A'-'Z' '0'-'9']*

rule toy_lang = parse
  | digit+ as inum
      { let num = int_of_string inum in
        let tok=INT num in
        memorize tok; tok
      
    }
  | digit+ '.' digit* as fnum
      { let num = float_of_string fnum in
      let tok=FLOAT num in
        memorize tok; tok
    }
  | id as word
      { try
        let token = Hashtbl.find keyword_table word in
        memorize token; token
      with Not_found ->
        let token=UNRESERVED word in
        memorize token; token
    }
  | '+'
  | '-'
  | '*'
  | '/' as op
      { let tok=OP op in
        memorize tok; tok
    }
  | [' ' '\t' '\n']    (* eat up whitespace *)
      ()
  | _ as c
      { 
      let tok=CHAR c in
        memorize tok; tok
    }
  | eof
      { raise End_of_file }

{
  let rec parse lexbuf =
     let token = toy_lang lexbuf in
     (* do nothing in this example *)
     parse lexbuf


}