let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds ((x | x) | y) | ((z | z) | y) = (y | (x | z)) | (y | (x | z))
let x, y, z be Element of L; :: thesis: ((x | x) | y) | ((z | z) | y) = (y | (x | z)) | (y | (x | z))
(y | (z | z)) | (y | (x | x)) = ((x | x) | y) | ((z | z) | y) by Th40;
hence ((x | x) | y) | ((z | z) | y) = (y | (x | z)) | (y | (x | z)) by Th65; :: thesis: verum