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 Th166:
  a -- (A\B) = (a--A) \ (a--B)
proof
  thus a -- (A\B) = a ++ ((--A)\(--B)) by Th17
    .= (a++--A) \ (a++--B) by Th150
    .= (a--A) \ (a--B);
end;
