let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | y) | (y | (x | z)) = y
let x, y, z be Element of L; :: thesis: (x | y) | (y | (x | z)) = y
z | x = x | z by Th20;
hence (x | y) | (y | (x | z)) = y by Th22; :: thesis: verum