let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for x, y being Element of L holds y | (((y | (x | x)) | (y | (x | x))) | (x | y)) = x | y
let x, y be Element of L; :: thesis: y | (((y | (x | x)) | (y | (x | x))) | (x | y)) = x | y
(x | y) | (y | (x | x)) = y by Th105;
hence y | (((y | (x | x)) | (y | (x | x))) | (x | y)) = x | y by Th102; :: thesis: verum