let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | (y | z)) | (x | z) = x
let x, y, z be Element of L; :: thesis: (x | (y | z)) | (x | z) = x
(z | z) | (y | z) = z by Th11;
hence (x | (y | z)) | (x | z) = x by Th15; :: thesis: verum