let A be QC-alphabet ; :: thesis: for p being Element of CQC-WFF A holds ('not' p) => (p => ('not' (VERUM A))) in TAUT A
let p be Element of CQC-WFF A; :: thesis: ('not' p) => (p => ('not' (VERUM A))) in TAUT A
p => (('not' p) => ('not' (VERUM A))) in TAUT A by CQC_THE1:43;
hence ('not' p) => (p => ('not' (VERUM A))) in TAUT A by Th15; :: thesis: verum