let A be QC-alphabet ; :: thesis: for p being Element of CQC-WFF A holds ('not' ('not' p)) => p in TAUT A
let p be Element of CQC-WFF A; :: thesis: ('not' ('not' p)) => p in TAUT A
( ('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 ('not' ('not' p)) => p in TAUT A by CQC_THE1:41, CQC_THE1:46; :: thesis: verum