proposition: Lemma nds_k_of_v n: nat_to_B (Nds.nds_k_of n) = nds_k_of (nat_to_B n).
proof: Proof. rewrite /Nds.nds_k_of /nds_k_of nat_to_B_ifle; Ytac h; Ytac0; clear h. case: n => /=. by rewrite cpred0. by move => n; rewrite (cpred_pr1 (CS_nat (nat_to_B_Nat n))). have <-: nat_to_B 9 = \9c. by rewrite /card_nine /card_eight/card_seven - nat_to_B6. rewrite - nat_to_B6 - nat_to_B5 - nat_to_B4. by rewrite - !nat_to_B_ifeq. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)