Module IndexedString <: IndexedType.
Definition t := string.
Definition eq := string_dec.
Fixpoint positive_of_digits (l : list bool) (p : positive) : positive :=
match l with
| [] => p
| b :: l' => if b then xI (positive_of_digits l' p) else xO (positive_of_digits l' p)
end.
Definition list_bool_of_ascii (a : ascii) : list bool :=
let (a0,a1,a2,a3,a4,a5,a6,a7) := a in
[a0; a1; a2; a3; a4; a5; a6; a7].
Fixpoint index (s : string) : positive :=
match s with
| EmptyString => 1
| String a s' => positive_of_digits (list_bool_of_ascii a) (index s')
end.

Lemma positive_of_digits_inj :
forall l l' p p',
List.length l = List.length l' ->
positive_of_digits l p = positive_of_digits l' p' ->
l = l' /\ p = p'.