proposition: Lemma val_olift m (i : 'I_m) : olift i = i.*2.+1 :> nat.
proof: Proof. case: m i => [[]//|m i]. rewrite -[LHS]/(i.*2.+1 %% (m.+1 + m.+1)) modn_small //. by rewrite addnS addSn !ltnS addnn leq_double -ltnS. Qed.
imports: mathcomp(all_ssreflect perm algebra.zmodp) mathcomp(zify)