let A be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF A holds (('not' p) => q) => ((q => p) => p) in TAUT A
let p, q be Element of CQC-WFF A; :: thesis: (('not' p) => q) => ((q => p) => p) in TAUT A
( (('not' ((q => p) => p)) => ((q => p) => p)) => ((q => p) => p) in TAUT A & ((('not' ((q => p) => p)) => ((q => p) => p)) => ((q => p) => p)) => ((('not' p) => q) => ((q => p) => p)) in TAUT A ) by Lm10, CQC_THE1:42;
hence (('not' p) => q) => ((q => p) => p) in TAUT A by CQC_THE1:46; :: thesis: verum