let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds x | ((y | y) | (z | (x | (x | y)))) = x | y
let x, y, z be Element of L; :: thesis: x | ((y | y) | (z | (x | (x | y)))) = x | y
(y | y) | (y | y) = y by Th21;
hence x | ((y | y) | (z | (x | (x | y)))) = x | y by Th47; :: thesis: verum