reserve i,j for Nat;

theorem
  for K being Ring,M being Matrix of K st -M=0.(K,len M,width M) holds
  M = 0.(K,len M,width M)
proof
  let K be Ring,M be Matrix of K;
  assume -M=0.(K,len M,width M);
  then --M = 0.(K,len M,width M) by Th9;
  hence thesis by Th1;
end;
