let A be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF A holds (p 'or' q) => (q 'or' p) in TAUT A
let p, q be Element of CQC-WFF A; :: thesis: (p 'or' q) => (q 'or' p) in TAUT A
(('not' p) => q) => (('not' q) => p) in TAUT A by LUKASI_1:31;
then (p 'or' q) => (('not' q) => p) in TAUT A by Lm1;
hence (p 'or' q) => (q 'or' p) in TAUT A by Lm1; :: thesis: verum