let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | y) | (x | (z | y)) = x
let x, y, z be Element of L; :: thesis: (x | y) | (x | (z | y)) = x
y | ((x | y) | y) = x | y by Th14;
hence (x | y) | (x | (z | y)) = x by Def1; :: thesis: verum