let p, q be Element of CQC-WFF ; :: thesis: (('not' p) => q) => (('not' q) => p) in TAUT
( (('not' p) => q) => (('not' q) => ('not' ('not' p))) in TAUT & (('not' q) => ('not' ('not' p))) => (('not' q) => p) in TAUT ) by Th26, Th29;
hence (('not' p) => q) => (('not' q) => p) in TAUT by Th3; :: thesis: verum