let A be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF A holds (p <=> q) => (q => p) in TAUT A
let p, q be Element of CQC-WFF A; :: thesis: (p <=> q) => (q => p) in TAUT A
p <=> q = (p => q) '&' (q => p) by QC_LANG2:def 4;
hence (p <=> q) => (q => p) in TAUT A by Th21; :: thesis: verum