Proof using name_map_bijective.
elim => //=.
move => n ns IH n' H_in H_in'.
case: H_in' => H_in'.
case: H_in.
left.
by rewrite H_in' tot_map_name_inverse_inv.
contradict H_in'.
apply: IH.
move => H_in'.
case: H_in.
by right.
Qed.

Lemma in_tot_map_msg :
forall m l n,
(forall nm, In nm l -> snd nm = m) ->
In (tot_map_name_inv n, m) l ->
In (n, tot_map_msg m) (map (fun nm : name * msg => (tot_map_name (fst nm), tot_map_msg (snd nm))) l).