let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y being Element of L holds (x | (y | x)) | y = y | y
let x, y be Element of L; :: thesis: (x | (y | x)) | y = y | y
set X = x;
set Y = y;
y | (x | (y | x)) = (x | (y | x)) | y by Th20;
hence (x | (y | x)) | y = y | y by Th32; :: thesis: verum