reserve T, X, Y for Subset of HP-WFF;
reserve p, q, r, s for Element of HP-WFF;

theorem Th3:
  (p => (q => r)) => ((p => q) => (p => r)) in CnPos (X)
proof
  T is Hilbert_theory & X c= T implies (p => (q => r)) => ((p => q) => (p
  => r)) in T;
  hence thesis by Def11;
end;
