let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y being Element of L holds (x | (((y | y) | x) | x)) | y = y | y
let x, y be Element of L; :: thesis: (x | (((y | y) | x) | x)) | y = y | y
set Y = y | y;
set X = x | y;
(y | y) | (y | ((x | y) | y)) = y by Th7;
hence (x | (((y | y) | x) | x)) | y = y | y by Th3; :: thesis: verum