let A be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF A holds (p => ('not' q)) => (q => ('not' p)) in TAUT A
let p, q be Element of CQC-WFF A; :: thesis: (p => ('not' q)) => (q => ('not' p)) in TAUT A
( (p => ('not' q)) => (('not' ('not' q)) => ('not' p)) in TAUT A & (('not' ('not' q)) => ('not' p)) => (q => ('not' p)) in TAUT A ) by Th26, Th28;
hence (p => ('not' q)) => (q => ('not' p)) in TAUT A by Th3; :: thesis: verum