let A be QC-alphabet ; :: thesis: for p being Element of CQC-WFF A holds p => ('not' ('not' p)) in TAUT A
let p be Element of CQC-WFF A; :: thesis: p => ('not' ('not' p)) in TAUT A
( ((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 p => ('not' ('not' p)) in TAUT A by A1, Th3; :: thesis: verum