reserve i,j for Nat;

theorem
  for K being Ring, M1, M2 being Matrix of K st len M1 = len M2 & width
  M1 = width M2 holds M1 = M1 - (M2 + -M2)
proof
  let K be Ring, M1, M2 be Matrix of K;
  assume len M1 = len M2 & width M1 = width M2;
  then M1 = M1 - (M2 - M2) by Th11;
  hence thesis;
end;
