theorem Th13:
  for M being Matrix of the carrier of V1 st len M = 0 holds Sum Sum M = 0.V1
  proof
    let M be Matrix of the carrier of V1;
    assume len M = 0;
    then len Sum M = 0 by MATRLIN:def 6;
    then Sum M = <*>(the carrier of V1);
    hence thesis by RLVECT_1:43;
  end;
