theorem Th33:
  p => (p '&' p) in IPC-Taut
proof
A1: (p => (p => (p '&' p))) => (p => (p '&' p)) in IPC-Taut by Th30;
  p => (p => (p '&' p)) in IPC-Taut by Def14;
  hence thesis by A1,Def14;
end;
