let L be non empty satisfying_DN_1 ComplLLattStr ; :: thesis: for x, y being Element of L holds (((x + y) ` ) + (y ` )) ` = y
let x, y be Element of L; :: thesis: (((x + y) ` ) + (y ` )) ` = y
((y ` ) + ((x + y) ` )) ` = y by Th18;
hence (((x + y) ` ) + (y ` )) ` = y by Th14; :: thesis: verum