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 Th27:
  p => 'not' 'not' p in TAUT(A)
proof
  (VERUM(A) => p) => ('not' p => 'not' VERUM(A)) in TAUT(A) &
('not' p => 'not'
  VERUM(A)) => 'not' 'not' p in TAUT(A) by Lm25,Th26;
  then
A1: (VERUM(A) => p) => 'not' 'not' p in TAUT(A) by Th3;
  p => (VERUM(A) => p) in TAUT(A) by Th5;
  hence thesis by A1,Th3;
end;
