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
  (F\G) -- r = (F--r) \ (G--r)
proof
  thus (F\G) -- r = --(r--(F\G)) by Th60
    .= --((r--F) \ (r--G)) by Th158
    .= (--(r--F)) \ (--(r--G)) by Th7
    .= (--(r--F)) \ (G--r) by Th60
    .= (F--r) \ (G--r) by Th60;
end;
