let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | (y | (y | (z | (x | y)))) = x | x
let x, y, z be Element of L; :: thesis: x | (y | (y | (z | (x | y)))) = x | x
x | (y | (x | x)) = x | x by Th12;
hence x | (y | (y | (z | (x | y)))) = x | x by Th57; :: thesis: verum