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

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