let Al be QC-alphabet ; :: thesis: for X being Subset of (CQC-WFF Al)
for p, q being Element of CQC-WFF Al holds X |- (p '&' q) => (q '&' p)
let X be Subset of (CQC-WFF Al); :: thesis: for p, q being Element of CQC-WFF Al holds X |- (p '&' q) => (q '&' p)
let p, q be Element of CQC-WFF Al; :: thesis: X |- (p '&' q) => (q '&' p)
(p '&' q) => (q '&' p) in Cn X by Th10;
hence X |- (p '&' q) => (q '&' p) by Def8; :: thesis: verum