let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y being Element of L holds x | ((((y | x) | (y | x)) | x) | x) = y | x
let x, y be Element of L; :: thesis: x | ((((y | x) | (y | x)) | x) | x) = y | x
((y | x) | (((y | x) | (y | x)) | (y | x))) | ((y | x) | (y | x)) = y | x by Th6;
hence x | ((((y | x) | (y | x)) | x) | x) = y | x by Th9; :: thesis: verum