let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | (y | (x | z))) | y = y | (z | x)
let x, y, z be Element of L; :: thesis: (x | (y | (x | z))) | y = y | (z | x)
z | x = x | z by Th20;
hence (x | (y | (x | z))) | y = y | (z | x) by Th28; :: thesis: verum