let X be Subset of CQC-WFF; :: thesis: for p being Element of CQC-WFF holds X |- (('not' p) => p) => p
let p be Element of CQC-WFF ; :: thesis: X |- (('not' p) => p) => p
(('not' p) => p) => p in Cn X by Th28;
hence X |- (('not' p) => p) => p by Def9; :: thesis: verum