let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | (y | z)) | (y | x) = x
let x, y, z be Element of L; :: thesis: (x | (y | z)) | (y | x) = x
z | y = y | z by Th20;
hence (x | (y | z)) | (y | x) = x by Th23; :: thesis: verum