let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | (y | ((z | z) | x)) = x | (y | z)
let x, y, z be Element of L; :: thesis: x | (y | ((z | z) | x)) = x | (y | z)
set X = z | z;
(z | z) | (z | z) = z by Th21;
hence x | (y | ((z | z) | x)) = x | (y | z) by Th61; :: thesis: verum