let X be Subset of CQC-WFF ; :: thesis: for p, q being Element of CQC-WFF holds (p '&' q) => (q '&' p) in Cn X
let p, q be Element of CQC-WFF ; :: thesis: (p '&' q) => (q '&' p) in Cn X
for T being Subset of CQC-WFF st T is being_a_theory & X c= T holds
(p '&' q) => (q '&' p) in T by Def1;
hence (p '&' q) => (q '&' p) in Cn X by Def2; :: thesis: verum