let A be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF A st ('not' p) => q is valid holds
('not' q) => p is valid
let p, q be Element of CQC-WFF A; :: thesis: ( ('not' p) => q is valid implies ('not' q) => p is valid )
assume A1: ('not' p) => q is valid ; :: thesis: ('not' q) => p is valid
(('not' p) => q) => (('not' q) => p) is valid ;
hence ('not' q) => p is valid by A1, CQC_THE1:65; :: thesis: verum