reserve E, x, y, X for set;
reserve A, B, C, D for Subset of E^omega;
reserve a, a1, a2, b, c, c1, c2, d, ab, bc for Element of E^omega;
reserve e for Element of E;
reserve i, j, k, l, n, n1, n2, m for Nat;

theorem Th11:
  a ^ a = a implies a = {}
proof
  assume a ^ a = a;
  then len a + len a = len a by AFINSQ_1:17;
  hence thesis;
end;
