let A be non empty set ; :: thesis: for v being Element of Valuations_in A
for p, q being Element of CQC-WFF
for J being interpretation of A holds J,v |= (p '&' q) => (q '&' p)
let v be Element of Valuations_in A; :: thesis: for p, q being Element of CQC-WFF
for J being interpretation of A holds J,v |= (p '&' q) => (q '&' p)
let p, q be Element of CQC-WFF ; :: thesis: for J being interpretation of A holds J,v |= (p '&' q) => (q '&' p)
let J be interpretation of A; :: thesis: J,v |= (p '&' q) => (q '&' p)
thus (Valid ((p '&' q) => (q '&' p)),J) . v = TRUE :: according to VALUAT_1:def 12 :: thesis: verum

proof

assume not (Valid ((p '&' q) => (q '&' p)),J) . v = TRUE ; :: thesis: contradiction
then A1: (Valid ((p '&' q) => (q '&' p)),J) . v = FALSE by XBOOLEAN:def 3;
(Valid ((p '&' q) => (q '&' p)),J) . v = (Valid ('not' ((p '&' q) '&' ('not' (q '&' p)))),J) . v by QC_LANG2:def 2
.= 'not' ((Valid ((p '&' q) '&' ('not' (q '&' p))),J) . v) by Th20
.= 'not' (((Valid (p '&' q),J) . v) '&' ((Valid ('not' (q '&' p)),J) . v)) by Th22
.= 'not' (((Valid (p '&' q),J) . v) '&' ('not' ((Valid (q '&' p),J) . v))) by Th20 ;
then A2: ((Valid (p '&' q),J) . v) '&' ('not' ((Valid (q '&' p),J) . v)) = TRUE by A1, MARGREL1:41;
then 'not' ((Valid (q '&' p),J) . v) = TRUE by MARGREL1:45;
then A3: (Valid (q '&' p),J) . v = FALSE by MARGREL1:41;
(Valid (p '&' q),J) . v = TRUE by A2, MARGREL1:45;
then ((Valid p,J) . v) '&' ((Valid q,J) . v) = TRUE by Th22;
hence contradiction by A3, Th22; :: thesis: verum

end;