reserve i,j for Nat;

theorem
  for K being Field, A,B being Matrix of K holds Indices (A+B)=Indices A
proof
  let K be Field,A,B be Matrix of K;
  len (A+B)=len A & width (A+B)=width A by MATRIX_3:def 3;
  hence thesis by MATRIX_4:55;
end;
