Set Warnings "-abstract-large-number".
Ltac invst H := inversion H; subst.
Ltac ctor := constructor.
Ltac ector := econstructor.
Ltac fext := apply functional_extensionality; intro.
Ltac gd x := generalize dependent x.
Ltac cg := try congruence.
Ltac clr_eqrefl:=
repeat match goal with
| H: ?A = ?A |- _ => clear H
end.
Ltac clr_dup:=
repeat match goal with
| H:?A, H':?A |- _ => clear H'
end.
Ltac clrs:=
clr_eqrefl; clr_dup.
Definition is_inj{T1 T2}(f:T1->T2):=
forall a b, f a = f b -> a = b.
Ltac ff_inj RR H := pose proof (ffx_eq_x_inj _ RR _ _ H); subst.
Fixpoint positive_len(x:positive):nat :=
match x with
| xH => O
| xI x0 => S (positive_len x0)
| xO x0 => S (positive_len x0)
end.
Fixpoint enc_v1(x:positive):positive :=
match x with
| xH => xH
| xI x0 => xI (xI (enc_v1 x0))
| xO x0 => xI (xO (enc_v1 x0))
end.
Definition enc_pair(x:positive*positive):positive :=
let (x1,x2):=x in
append (enc_v1 (Pos.of_succ_nat (positive_len x1))) (xO (append x1 x2)).

Lemma enc_pair_inj: is_inj enc_pair.