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)
A1: (p '&' q) => (q '&' p) in Cn X by Th31;
thus X |- (p '&' q) => (q '&' p) by A1, Def9; :: thesis: verum