reserve E, x, y, X for set;
reserve A, B, C for Subset of E^omega;
reserve a, b for Element of E^omega;
reserve i, k, l, kl, m, n, mn for Nat;

theorem Th78:
  x in A? iff x = <%>E or x in A
proof
  x in A? iff x in {<%>E} \/ A by Th76;
  then x in A? iff x in {<%>E} or x in A by XBOOLE_0:def 3;
  hence thesis by TARSKI:def 1;
end;
