(* ** Increment *)
(* In the loop, we assume that we are at a certain bit. We stop if we reached the HSB *)
Definition Increment_Step_Rel : pRel sigPos^+ (option unit) 1 :=
fun tin '(yout, tout) =>
(forall (p : positive) (b : bool) (bits : list bool),
atBit tin[@Fin0] p b bits ->
match b, yout with
| true, None => (* The current symbol is [true]; we simply flip it and repeat the loop *)
movedToLeft tout[@Fin0] p false bits
| false, Some tt => (* The current symbol is [false]; we flip it (to [true]) and move to the HSB and terminate *)
atHSB tout[@Fin0] (append_bits p (true :: bits)) /\ yout = Some tt
| _, _ => False
end) /\
(forall (p : positive), (* If we already are at the HSB, push a 1 and stop *)
atHSB tin[@Fin0] p ->
atHSB tout[@Fin0] (pushHSB p false) /\ yout = Some tt).
(* Compute append_bits 1 [false; true; true]. *)
(* Compute pushHSB (append_bits 1 [false; true; true]) false. *)
Definition Increment_Step : pTM sigPos^+ (option unit) 1 :=
Switch ReadPosSym
(fun (s : option bool) =>
match s with
| Some true  => Return (SetBitAndMoveLeft false) None (* b = true *)
| Some false => Return (SetBitAndMoveLeft true;; GoToHSB) (Some tt) (* b = false *)
| None => Return (PushHSB false) (Some tt) (* HSB *)
end).

Lemma pushHFS_append2 bits :
pushHSB (append_bits 2 bits) false = append_bits 4 bits.