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

theorem Th21: :: Hypothetical syllogism
  (p => q) => ((q => r) => (p => r)) in HP_TAUT
proof
  (q => r) => ((p => q) => (p => r)) in HP_TAUT by Th19;
  hence thesis by Th20;
end;
