Combining different rules in ocamllex

When I compile the basic_lexer.mll file below, and type 
Basic_lexer.parse_all   "abc\"def\"ghi";; in the interpreter,
I am epecting to get 
[Basic_lexer.CHAR 'a'; Basic_lexer.CHAR 'b'; Basic_lexer.CHAR 'c';
 Basic_lexer.STRING "def";
 Basic_lexer.CHAR 'g'; Basic_lexer.CHAR 'h'; Basic_lexer.CHAR 'i']
 
 but instead I get
 
[Basic_lexer.CHAR 'a'; Basic_lexer.CHAR 'b'; Basic_lexer.CHAR 'c';
 Basic_lexer.CHAR 'e'; Basic_lexer.CHAR 'f'; Basic_lexer.CHAR 'h';
 Basic_lexer.CHAR 'i']

 

 What did I do wrong ?
 

 Contents of basic_lexer.mll file :
 

 {

type lexeme=
    CHAR of char
   |STRING of string;;
    
let list_accu=ref[]
let string_accu=ref""

let add_to_list x=(list_accu:=x::(!list_accu)) 
let add_to_string c=(string_accu:=(!string_accu)^(String.make 1 c))    
let finish_quote ()=(add_to_list (STRING(!string_accu)); string_accu:=""; );;
    
}
    

rule usual = parse
  | '"'  { quoted_string lexbuf;}
  | _ as c {add_to_list (CHAR c);}
  | eof { raise End_of_file }
and quoted_string=parse
  | "\\\"" {add_to_string '\''}
  | "\\\\" {add_to_string '\\'}
  | '"' {finish_quote(); usual lexbuf}
  | _ as c {add_to_string c}
  |eof {raise End_of_file}     
  
{



let parse_all s =
          let _=(list_accu:=[];string_accu:="") in
          try
            let lexbuf = Lexing.from_string s in
            let _=(while true do
             usual lexbuf 
            done) in
            []
          with End_of_file ->
            List.rev(!list_accu)


  
}