let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | (y | z) = x | (z | y)
let x, y, z be Element of L; :: thesis: x | (y | z) = x | (z | y)
(z | (x | (z | y))) | x = x | (z | y) by Th28;
hence x | (y | z) = x | (z | y) by Th30; :: thesis: verum