let Al be QC-alphabet ; :: thesis: for p being Element of CQC-WFF Al holds (('not' p) => p) => p is valid
let p be Element of CQC-WFF Al; :: thesis: (('not' p) => p) => p is valid
(('not' p) => p) => p in TAUT Al

proof

TAUT Al is being_a_theory by Th11;
hence (('not' p) => p) => p in TAUT Al ; :: thesis: verum

end;

hence (('not' p) => p) => p is valid ; :: thesis: verum