let L be non empty satisfying_DN_1 ComplLLattStr ; :: thesis: for x, y, z being Element of L holds (((((x + (y ` )) + z) ` ) + y) ` ) ` = y
let x, y, z be Element of L; :: thesis: (((((x + (y ` )) + z) ` ) + y) ` ) ` = y
(((((x + (y ` )) + z) ` ) + y) ` ) + (((y ` ) + y) ` ) = ((((x + (y ` )) + z) ` ) + y) ` by Th32;
hence (((((x + (y ` )) + z) ` ) + y) ` ) ` = y by Th36; :: thesis: verum