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 Th25:
  'not' 'not' p => p in TAUT(A)
proof
  'not' 'not' p => ('not' p => 'not' VERUM(A)) in TAUT(A) & ('not' p =>
'not'
  VERUM(A)) => (VERUM(A) => p) in TAUT(A) by Lm23,Th24;
  then 'not' 'not' p => (VERUM(A) => p) in TAUT(A) by Th3;
  then VERUM(A) => ('not' 'not' p => p) in TAUT(A) by Th15;
  hence thesis by CQC_THE1:41,46;
end;
