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

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