proposition: Lemma row_mx_rbehead (x : 'rV_(1 + n)) : row_mx (\row__ (x ``_ 0)) (rbehead x) = x.
proof: Proof. apply/rowP => i; rewrite mxE; case: splitP=> [j|j i1j]. - rewrite (ord1 j) mxE => i0; congr (x _ _); exact/val_inj. - rewrite mxE; congr (x _ _); exact/val_inj. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup finalg perm zmodp) mathcomp(matrix mxalgebra vector) ssr_ext f2