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

theorem Th6:
  'not' ( p 'or' q ) => ( 'not' p '&' 'not' q ) in TAUT(A)
proof
  'not' ( p 'or' q ) = 'not' 'not' ( 'not' p '&' 'not' q ) by QC_LANG2:def 3;
  hence thesis by LUKASI_1:25;
end;
