let X be Subset of CQC-WFF; :: thesis: for p, q being Element of CQC-WFF holds X |- (p '&' q) => (q '&' p)
let p, q be Element of CQC-WFF ; :: thesis: X |- (p '&' q) => (q '&' p)
(p '&' q) => (q '&' p) in Cn X by Th31;
hence X |- (p '&' q) => (q '&' p) by Def9; :: thesis: verum