Section FixM.
Variable Σ : finType.
Let n := (projT1 (finite_n Σ)).
Let f := (projT1 (projT2 (finite_n Σ))).
Let g := (proj1_sig (projT2 (projT2 (finite_n Σ)))).
Let Hf := (proj1 (proj2_sig (projT2 (projT2 (finite_n Σ))))).
Let Hg := (proj2 (proj2_sig (projT2 (projT2 (finite_n Σ))))).
Instance R : Retract (Fin.t n) Σ.
Definition encode_tape' (t : tape Σ) : tape bool := encode_tape (mapTape f t).

Lemma ReadB_Realise' :
ReadB n ⊨ fun t '(l, t') => forall t_sig : tape Σ, t = [| encode_tape' t_sig |] -> t' = t /\ l = map_opt f (current t_sig).