reserve i,n for Nat,
  K for Field,
  M1,M2,M3,M4 for Matrix of n,K;

theorem
  Trace (M1+-M1)=0.K
proof
  len M1=n & width M1=n by MATRIX_0:24;
  then Trace (M1+(-M1))=Trace (0.(K,n,n)) by MATRIX_4:2
    .=Trace (0.(K,n))
    .=0.K by Th55;
  hence thesis;
end;
