let Al be QC-alphabet ; :: thesis: for p being Element of CQC-WFF Al holds (('not' p) => p) => p in TAUT Al
let p be Element of CQC-WFF Al; :: thesis: (('not' p) => p) => p in TAUT Al
TAUT Al is being_a_theory by Th15;
hence (('not' p) => p) => p in TAUT Al by Def1; :: thesis: verum