let Al be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF Al holds (p '&' q) => (q '&' p) is valid
let p, q be Element of CQC-WFF Al; :: thesis: (p '&' q) => (q '&' p) is valid
(p '&' q) => (q '&' p) in TAUT Al

proof

TAUT Al is being_a_theory by Th11;
hence (p '&' q) => (q '&' p) in TAUT Al ; :: thesis: verum

end;

hence (p '&' q) => (q '&' p) is valid ; :: thesis: verum