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