(* fonctions réciproques gauche et droite de iad_conv *)
Inductive ad_couple : Set :=
cpla : ad -> ad -> ad_couple.
Fixpoint iad_conv_inv_0 (p : positive) : ad_couple :=
match p with
| xH => cpla (Npos 1) N0
| xO xH => cpla N0 (Npos 1)
| xI xH => cpla (Npos 1) (Npos 1)
| xO (xO p') =>
match iad_conv_inv_0 p' with
| cpla N0 N0 => cpla N0 N0
| cpla N0 (Npos p1) => cpla N0 (Npos (xO p1))
| cpla (Npos p0) N0 => cpla (Npos (xO p0)) N0
| cpla (Npos p0) (Npos p1) => cpla (Npos (xO p0)) (Npos (xO p1))
end
| xO (xI p') =>
match iad_conv_inv_0 p' with
| cpla N0 N0 => cpla N0 (Npos 1)
| cpla N0 (Npos p1) => cpla N0 (Npos (xI p1))
| cpla (Npos p0) N0 => cpla (Npos (xO p0)) (Npos 1)
| cpla (Npos p0) (Npos p1) => cpla (Npos (xO p0)) (Npos (xI p1))
end
| xI (xO p') =>
match iad_conv_inv_0 p' with
| cpla N0 N0 => cpla (Npos 1) N0
| cpla N0 (Npos p1) => cpla (Npos 1) (Npos (xO p1))
| cpla (Npos p0) N0 => cpla (Npos (xI p0)) N0
| cpla (Npos p0) (Npos p1) => cpla (Npos (xI p0)) (Npos (xO p1))
end
| xI (xI p') =>
match iad_conv_inv_0 p' with
| cpla N0 N0 => cpla (Npos 1) (Npos 1)
| cpla N0 (Npos p1) => cpla (Npos 1) (Npos (xI p1))
| cpla (Npos p0) N0 => cpla (Npos (xI p0)) (Npos 1)
| cpla (Npos p0) (Npos p1) => cpla (Npos (xI p0)) (Npos (xI p1))
end
end.
Definition iad_conv_inv (a : ad) : ad_couple :=
match a with
| N0 => cpla N0 N0
| Npos p => iad_conv_inv_0 p
end.

Lemma iad_inv_inv_0 :
forall a0 a1 : ad, iad_conv_inv (iad_conv a0 a1) = cpla a0 a1.