reserve x for set;
reserve k, l for Nat;
reserve p, q for FinSequence;
reserve R for Relation;
reserve p, q for RedSequence of R;
reserve E for set;
reserve s, t for XFinSequence;
reserve p, q for XFinSequence-yielding FinSequence;
reserve E for set;
reserve S, T, U for semi-Thue-system of E;
reserve s, t, s1, t1, u, v, u1, v1, w for Element of E^omega;
reserve p for FinSequence of E^omega;

theorem Th50:
  Lang(w, {}(E^omega, E^omega)) = {w}
proof
  for x holds not(x <> w & x in Lang(w, {}(E^omega, E^omega))) by Th46,Th42;
  then for x be object holds x in Lang(w, {}(E^omega, E^omega)) iff x = w
  by Th47;
  hence thesis by TARSKI:def 1;
end;
