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 Th15;
hence (p '&' q) => (q '&' p) in TAUT Al by Def1; :: thesis: verum

end;

hence (p '&' q) => (q '&' p) is valid by Lm13; :: thesis: verum