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

theorem
  (p '&' q) in IPC-Taut iff (q '&' p) in IPC-Taut
proof
  hereby
    assume
A1: p '&' q in IPC-Taut;
    then
A2: q in IPC-Taut by Th34;
    p in IPC-Taut by A1,Th34;
    hence q '&' p in IPC-Taut by A2,Th34;
  end;
  assume
A3: q '&' p in IPC-Taut;
  then
A4: p in IPC-Taut by Th34;
  q in IPC-Taut by A3,Th34;
  hence thesis by A4,Th34;
end;
