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 Th53;
  hence thesis by Th53;
end;
