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 Th158:
  r -- (F\G) = (r--F) \ (r--G)
proof
  thus r -- (F\G) = r ++ ((--F)\(--G)) by Th7
    .= (r++--F) \ (r++--G) by Th139
    .= (r--F) \ (r--G);
end;
