let L be non empty satisfying_DN_1 ComplLattStr ; :: thesis: for x, y being Element of L holds x + y = y + x
let x, y be Element of L; :: thesis: x + y = y + x
x + y = ((x + y) ` ) ` by Th23
.= y + x by Th25 ;
hence x + y = y + x ; :: thesis: verum