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 Th28:
  ('not' 'not' p => q) => (p => q) in TAUT(A) & (p => q) => ('not'
  'not' p => q) in TAUT(A)
proof
  p => 'not' 'not' p in TAUT(A) by Th27;
  hence ('not' 'not' p => q) => (p => q) in TAUT(A) by Th2;
  'not' 'not' p => p in TAUT(A) by Th25;
  hence thesis by Th2;
end;
