theorem
  for M being Matrix of 1,D holds M@ = M
  proof
    let M be Matrix of 1,D;
A1: M = <* <* M*(1,1) *> *> by MATRIX13:20;
    Indices M = [: {1},{1} :] by MATRIX_0:24,FINSEQ_1:2;
    then [1,1] in Indices M by ZFMISC_1:28;
    then (M@)*(1,1) = M*(1,1) by MATRIX_0:def 6;
    hence thesis by A1,MATRIX13:20;
  end;
