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

theorem Th34:
  p => q in TAUT(A) iff 'not' q => 'not' p in TAUT(A)
proof
  (p => q) => ('not' q => 'not' p) in TAUT(A) by Th26;
  hence p => q in TAUT(A) implies 'not' q => 'not' p in TAUT(A) by CQC_THE1:46;
  ('not' q => 'not' p) => (p => q) in TAUT(A) by Th24;
  hence 'not' q => 'not' p in TAUT(A) implies p => q in TAUT(A) by CQC_THE1:46;
end;
