proposition: Lemma row_mx_rbelast (x : 'rV_n.+1) : castmx (erefl 1%N, addn1 n) (row_mx (rbelast x) (\row__ (rlast x))) = x.
proof: Proof. apply/rowP => i; rewrite castmxE /= mxE; case: splitP => [j /= ij|k /=]. rewrite mxE; congr (x _ _); exact/val_inj. rewrite (ord1 k) addn0 => ni; rewrite mxE; congr (x _ _); exact/val_inj. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup finalg perm zmodp) mathcomp(matrix mxalgebra vector) ssr_ext f2