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

theorem Th56:
  (p 'or' q) => (q 'or' p) in IPC-Taut
proof
A1: p => (q 'or' p) in IPC-Taut by Def14;
A2: q => (q 'or' p) in IPC-Taut by Def14;
  (p => (q 'or' p)) => ((q => (q 'or' p)) => ((p 'or' q) => (q 'or' p)))
  in IPC-Taut by Def14;
  then ((q => (q 'or' p)) => ((p 'or' q) => (q 'or' p))) in IPC-Taut by A1
,Def14;
  hence thesis by A2,Def14;
end;
