reserve w, w1, w2 for Element of ExtREAL;
reserve c, c1, c2 for Complex;
reserve A, B, C, D for complex-membered set;
reserve F, G, H, I for ext-real-membered set;
reserve a, b, s, t, z for Complex;
reserve f, g, h, i, j for ExtReal;
reserve r for Real;
reserve e for set;

theorem
  (a+b)++A = a++(b++A)
proof
  thus (a+b)++A = {a}++{b}++A by Th51
    .= a++(b++A) by Th50;
end;
