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 | (x | z))) | y)
let x, y, z be Element of L; :: thesis: x | ((y | (y | (z | x))) | x) = y | ((x | (y | (x | z))) | y)
set Z = y | (x | z);
(x | (y | (x | z))) | y = y | (z | x) by Th30;
hence x | ((y | (y | (z | x))) | x) = y | ((x | (y | (x | z))) | y) by Th53; :: thesis: verum