theorem
  A@ = <* <* A*(1,1) *>, <* A*(1,2) *>, <* A*(1,3) *> *>
  proof
A1: A@ is Matrix of 3,1,D by Th23;
    [1,1] in Indices A & [1,2] in Indices A & [1,3] in Indices A
      by MATRIX_0:31;
    then (A@)*(1,1) = A*(1,1) & (A@)*(2,1) = A*(1,2) & (A@)*(3,1) = A*(1,3)
      by MATRIX_0:def 6;
    hence thesis by A1,Th27;
  end;
