let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | ((x | y) | (z | y)) = x | y
let x, y, z be Element of L; :: thesis: x | ((x | y) | (z | y)) = x | y
(x | y) | (x | (z | y)) = x by Th15;
hence x | ((x | y) | (z | y)) = x | y by Th16; :: thesis: verum