let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | ((y | z) | x)) | (y | y) = y
let x, y, z be Element of L; :: thesis: (x | ((y | z) | x)) | (y | y) = y
(y | z) | (y | y) = y by Th21;
hence (x | ((y | z) | x)) | (y | y) = y by Th51; :: thesis: verum