reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;

theorem Th24: :: Hypothetical syllogism
  (p => q) => ((q => r) => (p => r)) in IPC-Taut
proof
  (q => r) => ((p => q) => (p => r)) in IPC-Taut by Th22;
  hence thesis by Th23;
end;
