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

theorem Th54:
  (p 'or' p) => p in IPC-Taut
proof
A1: p => p in IPC-Taut by Th17;
  (p => p) => ((p => p) => ((p 'or' p) => p)) in IPC-Taut by Def14;
  then (p => p) => ((p 'or' p) => p) in IPC-Taut by A1,Def14;
  hence thesis by A1,Def14;
end;
