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)))) = x | (y | (x | x))
let x, y, z be Element of L; :: thesis: x | (y | (y | (z | (x | y)))) = x | (y | (x | x))
y | (x | (y | (y | (z | (x | y))))) = y | (x | x) by Th56;
hence x | (y | (y | (z | (x | y)))) = x | (y | (x | x)) by Th27; :: thesis: verum