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

theorem
  Trace (M1-M2-M3)=Trace M1-Trace M2-Trace M3
proof
  Trace (M1-M2-M3)=Trace (M1-M2)-Trace M3 by Th59
    .=Trace M1-Trace M2-Trace M3 by Th59;
  hence thesis;
end;
