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