reserve A for QC-alphabet;
reserve p, q, r, s for Element of CQC-WFF(A);

theorem Th21:
  ( p '&' q ) => q in TAUT(A)
proof
  ( q '&' p ) => q in TAUT(A) & ( p '&' q ) => ( q '&' p ) in TAUT(A) by Th19,
CQC_THE1:45;
  hence thesis by LUKASI_1:3;
end;
