let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | (y | (x | (z | (y | x)))) = x | x
let x, y, z be Element of L; :: thesis: x | (y | (x | (z | (y | x)))) = x | x
(x | (y | (x | (z | (y | x))))) | (x | (z | (y | x))) = x by Th31;
hence x | (y | (x | (z | (y | x)))) = x | x by Th18; :: thesis: verum