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

theorem
  p in IPC-Taut or q in IPC-Taut implies (p 'or' q) in IPC-Taut
proof
  assume
A1: p in IPC-Taut or q in IPC-Taut;
  now
    per cases by A1;
    case
A2:   p in IPC-Taut;
      p => (p 'or' q) in IPC-Taut by Def14;
      hence thesis by A2,Def14;
    end;
    case
A3:   q in IPC-Taut;
      q => (p 'or' q) in IPC-Taut by Def14;
      hence thesis by A3,Def14;
    end;
  end;
  hence thesis;
end;
