reserve i,j for Nat;

theorem
  for K being Ring,M1,M2,M3 being Matrix of K st -M1=-M2 holds M1=M2
proof
  let K be Ring,M1,M2,M3 be Matrix of K;
  assume -M1=-M2;
  then --M1=M2 by Th1;
  hence thesis by Th1;
end;
