theorem Th13:
  for M be 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 Def6;
  then Sum M = <*>(the carrier of V1);
  hence thesis by RLVECT_1:43;
end;
