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

theorem Th25:
  p => q in IPC-Taut implies (q => r) => (p => r) in IPC-Taut
proof
  assume
A1: p => q in IPC-Taut;
  (p => q) => ((q => r) => (p => r)) in IPC-Taut by Th24;
  hence thesis by A1,Def14;
end;
