let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | y) | (y | (z | x)) = y
let x, y, z be Element of L; :: thesis: (x | y) | (y | (z | x)) = y
set Y = x | ((y | x) | x);
x | ((y | x) | x) = x | y by Th19;
hence (x | y) | (y | (z | x)) = y by Def1; :: thesis: verum