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 Th8:
  -- (F \+\ G) = (--F) \+\ (--G)
proof
  thus -- (F \+\ G) = --(F\G) \/ --(G\F) by Th5
    .= --F \ --G \/ --(G\F) by Th7
    .= (--F) \+\ --G by Th7;
end;
