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

theorem Th27:
  (p => (q => r)) => ((s => q) => (p => (s => r))) in IPC-Taut
proof
A1: (((q => r) => (s => r)) => (p => (s => r))) => ((s => q) => (p => (s =>
  r))) in IPC-Taut by Lm4;
  ((((q => r) => (s => r)) => (p => (s => r))) => ((s => q) => (p => (s =>
r)))) => ((p => (q => r)) => ((s => q) => (p => (s => r)))) in IPC-Taut by Lm4;
  hence thesis by A1,Def14;
end;
