let A be set ; :: thesis: for a, b being Element of (NormForm A) holds a "/\" b = b "/\" a

set G = NormForm A;

let a, b be Element of (NormForm A); :: thesis: a "/\" b = b "/\" a

reconsider a9 = a, b9 = b as Element of Normal_forms_on A by Def12;

a "/\" b = mi (a9 ^ b9) by Def12

.= mi (b9 ^ a9) by Th48

.= b "/\" a by Def12 ;

hence a "/\" b = b "/\" a ; :: thesis: verum

set G = NormForm A;

let a, b be Element of (NormForm A); :: thesis: a "/\" b = b "/\" a

reconsider a9 = a, b9 = b as Element of Normal_forms_on A by Def12;

a "/\" b = mi (a9 ^ b9) by Def12

.= mi (b9 ^ a9) by Th48

.= b "/\" a by Def12 ;

hence a "/\" b = b "/\" a ; :: thesis: verum