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

theorem Th17:
  p => p in IPC-Taut
proof
A1: p => (p => p) in IPC-Taut by Def14;
A2: p => ((p => p) => p) in IPC-Taut by Def14;
  (p => ((p => p) => p)) => ((p => (p => p)) => (p => p)) in IPC-Taut by Def14;
  then (p => (p => p)) => (p => p) in IPC-Taut by A2,Def14;
  hence thesis by A1,Def14;
end;
