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 Th26:
  (p => q) => ('not' q => 'not' p) in TAUT(A)
proof
  'not' q => (q => 'not' VERUM(A)) in TAUT(A) &
(q => 'not' VERUM(A)) => ((p
=> q)
  => ( p => 'not' VERUM(A))) in TAUT(A) by Lm23,Th9;
  then
A1: 'not' q => ((p => q) => (p => 'not' VERUM(A))) in TAUT(A) by Th3;
  'not' q => ((p => 'not' VERUM(A)) => 'not' p) in TAUT(A) by Lm25,Th13;
  then 'not' q => ((p => q) => 'not' p) in TAUT(A) by A1,Th22;
  hence thesis by Th15;
end;
